Matrix-isolation FT-IR studies and ab initio calculations of hydrogen-bonded complexes of molecules modeling cytosine or isocytosine tautomers. 6. Experimental observation of a water-induced tautomeric shift for 2-hydroxypyrimidine and 5-bromo-2-hydroxypyrimidine

Article abstract of DOI:10.1021/jp981815l

The H-bond interaction of the cytosine model compound 2-hydroxypyrimidine and its 5-bromo derivative with water is investigated using the combined matrix-isolation FT-IR and theoretical ab initio method. As predicted by the ab initio calculations, both compounds occur dominantly in the hydroxy tautomeric forms. The estimated K_T(h/o) values are 60 and 184, respectively. When water is added to the Ar matrix, a noticeable shift of the tautomeric equilibrium towards the oxo form is observed. The theoretical results indicate that the closed N...H-O(H)...H-O and C=O...H-O(H)...H-N H-bonded water complexes are the most stable systems for the hydroxy and the oxo tautomers, respectively. The experimental spectra are consistent with this prediction, but additional structures, such as an open N...H-OH complex of the hydroxy tautomer, are also identified. The frequency shift of the stretching mode of doubly H-bonded water in the two closed complexes is larger, and the ratio between the calculated and measured frequencies smaller than expected from the correlation established before for open, singly H-bonded complexes involving similar molecules. Although some cooperativity exists between the two H bonds in each of the closed complexes, this effect is limited because the geometrical structures of both H bonds are noticeably perturbed from the perfect alignment due to the cyclic arrangement of the complex. A possible mechanism of the proton transfer process leading from the hydroxy to the oxo tautomeric form is discussed in terms of proton tunneling and in relation to recent literature data.

Full text of DOI:10.1021/jp981815l

J. Phys. Chem. A 1998, 102, 8157-8168
8157
Matrix-Isolation FT-IR Studies and ab Initio Calculations of Hydrogen-Bonded Complexes
of Molecules Modeling Cytosine or Isocytosine Tautomers. 6. Experimental Observation of
a Water-Induced Tautomeric Shift for 2-Hydroxypyrimidine and
5-Bromo-2-hydroxypyrimidine
Johan Smets,† Alain Destexhe,† Ludwik Adamowicz,*,‡ and Guido Maes*,†,§
Department of Chemistry, UniVersity of LeuVen, Celestijnenlaan 200F, B-3001, HeVerlee, Belgium, and
Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721
ReceiVed: April 10, 1998
The H-bond interaction of the cytosine model compound 2-hydroxypyrimidine and its 5-bromo derivative
with water is investigated using the combined matrix-isolation FT-IR and theoretical ab initio method. As
predicted by the ab initio calculations, both compounds occur dominantly in the hydroxy tautomeric forms.
The estimated KT(h/o) values are 60 and 184, respectively. When water is added to the Ar matrix, a noticeable
shift of the tautomeric equilibrium towards the oxo form is observed. The theoretical results indicate that the
closed N‚‚‚H-O(H)‚‚‚H-O and CdO‚‚‚H-O(H)‚‚‚H-N H-bonded water complexes are the most stable
systems for the hydroxy and the oxo tautomers, respectively. The experimental spectra are consistent with
this prediction, but additional structures, such as an open N‚‚‚H-OH complex of the hydroxy tautomer, are
also identified. The frequency shift of the stretching mode of doubly H-bonded water in the two closed
complexes is larger, and the ratio between the calculated and measured frequencies smaller than expected
from the correlation established before for open, singly H-bonded complexes involving similar molecules.
Although some cooperativity exists between the two H bonds in each of the closed complexes, this effect is
limited because the geometrical structures of both H bonds are noticeably perturbed from the perfect alignment
due to the cyclic arrangement of the complex. A possible mechanism of the proton transfer process leading
from the hydroxy to the oxo tautomeric form is discussed in terms of proton tunneling and in relation to
recent literature data.
Introduction
to 2-hydroxypyridine and 4-hydroxypyrimidine, which also have
the OH group vicinal to the ring nitrogen atom, these compounds
occur predominantly as the hydroxy tautomers in Ar.6 The pres-
ence of two or more tautomeric forms in comparable amounts,
as for 2-hydroxypyridine7-9 or for cytosine derivatives,10-13
complicates the spectral assignment of the experimental absorp-
tions, because water complexes of all these tautomers have to
be considered.
Some interesting studies on 2-hydroxypyridine, its water
complexes, and the water-enhanced tautomeric process in this
compound in both the ground and the electronically excited
states have recently appeared.14-21 Also a number of theoretical
and experimental studies on proton-transfer processes occurring
along intra- and intermolecular H bonds in 2-hydroxypyridine
and its water complexes in the ground and electronically and
vibrationally excited states have been published recently.22-27
In particular, Sobolewski et al. calculated the barrier height for
the proton-transfer process in 2HPM in the ground and the
electronically excited state using the CASSCF and CASPT2 ab
initio methods.26,27 This work is particularly relevant to the
present study.
In the former papers of this series, hydrogen bonding of the
cytosine model molecules pyridine/pyrimidine,1 4-aminopyri-
dine/4-amionopyrimidine,2 3-hydroxypyridine/4-hydroxypyri-
dine,3 1-methyl-2-pyrimidone, and 1,9,9-trimethylcytosine4 with
water in Ar matrices has been described based on results
obtained using a combined IR experimental and ab initio
theoretical approach. This was a preliminary step in the
investigation aimed at characterization of the H-bond interaction
of the biologically more important cytosines and isocytosines
with water in isolated conditions. For all the model compounds
mentioned above, only a single tautomeric form is present in
Ar matrices. In the following step, the combined experimental
and ab initio results allowed us to interpret the matrix FT-IR
spectra of 1-methylcytosine in water-doped Ar matrices.5 We
assigned the spectral features obtained for this system as
originating from two different H-bonded water complexes of
the amino-oxo tautomer, and one water complex of the imino-
oxo (E) tautomer.
Following the 1-methylcytosine-water study, in the present
work we describe the H-bond interaction of 2-hydroxypyrimi-
dine (2HPM) and 5-bromo-2-hydroxypyrimidine (5B2HPM) and
their oxo tautomers, 2-pyrimidone (2OPM) and 5-bromo-2-
pyrimidone (5B2OPM), with water in Ar matrices. Contrary
In the present work we demonstrate that the relative concen-
tration of the hydroxy and oxo tautomers of 2HPM and
5B2HPM changes noticeably upon doping the isolating matrix
with water. Such a solvent-assisted tautomeric transition from
the hydroxy to the oxo form by a double proton-transfer process
has been suggested to occur in matrices by formation of self-
associates and their rearrangement involving internal hydrogen
transfer.6b The shift of the tautomeric equilibrium induced by
* Corresponding authors.
† University of Leuven.
‡ University of Arizona.
§ Senior Research Associate of the Belgian National Fund for Scientific
Research.
S1089-5639(98)01815-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/29/1998

8158 J. Phys. Chem. A, Vol. 102, No. 42, 1998
Smets et al.
TABLE 1: Total (au) and Relative (kJ‚mol-1) Energies of the Hydroxy and Oxo Tautomers of 2-Hydroxypyrimidine and the
5-Halogeno Derivatives Calculated with the 6-31++G** Basis Set or the BBS Basis Seta
X ) H
X ) Brb
X ) Cl
X ) F
method
hydroxy
oxo
hydroxy
oxo
hydroxy
oxo
hydroxy
oxo
SCF
-337.5874643 -337.5820415 -2909.3516372 -2909.3434037 -796.4817159 -796.4731843 -436.4329028 -436.4229334
-338.6131794 -338.6063259 2910.6275222 -2910.6178818 -797.6324585 -797.6225754 -437.6236118 -437.6116968
MP2b
ZPEc
total
0.0883348
0.0882529
0.06989013
0.06985296
0.07046946
0.07043562
0.07165755
0.07159896
-338.5248446 -338.5180735 -2910.557632 -2910.548028 -747.561989 -747.5521398 -437.5519543 -437.5400978
d
∆ET
0.00
3.07
17.79
6.82
0.00
1.81
26350
25.22
5.46
0.00
1.72
33914
25.86
5.29
0.00
1.78
284201
31.13
5.26
µ (D)
KT(298.15 K)e 1308
a Geometries optimized at the SCF/6-31++G** or BBS level of theory (X is the substituent at the C5 position). b Only valence correlation is
considered. c Calculated as 0.9∑hνi. d Energy difference between tautomers of one compound. e -RT ln KT = DET. f BBS basis set used (see text).
SCHEME 1
environment is usually explained by the difference in the
dipole-dipole or dipole-induced dipole interactions of the
tautomers with the solvent molecules. The dominance of the
oxo tautomer in the polar environment is thus explained by its
larger dipole-dipole interaction with solvent molecules than
for the hydroxy tautomer. This interaction shifts the tautomeric
equilibrium from the hydroxy form to the oxo form. This is
also the reason why 2HPM occurs as the oxo tautomer in the
crystalline state and in polar solvents,28,29 but as the hydroxy
tautomer in the gas phase and in Ar matrices.30
The ab initio assisted analysis of the FT-IR spectra of 2HPM
and 5B2HPM in water-doped matrices presented in this work
provides an insight into the H-bond properties of hydroxy
tautomers of heterocyclic compounds with the OH group vicinal
to the nitrogen atom in the pyrimidine ring. In particular, the
present results will allow in the future investigations to
discriminate between the H-bonded complexes formed by the
hydroxy and oxo tautomers of cytosine derivatives.13
Methodology
Experimental Method. A detailed description of the
experimental method used here has been presented else-
where.31,32 Optimal sublimation temperatures were 55 and 75
°C for 2HPM and 5B2HPM, respectively, at the Ar deposition
rate of 5 mmol/h. Similar to all our earlier studies on water
complexes in Ar,1-5 the water-to-base ratio varying between
1:1 and 5:1 was used. The latter ratio ensures an excess amount
of the 1:1 complex species to be present in the matrix with only
weak spectral manifestations of higher-stoichiometry complexes.
2HPM was obtained from 2HPM‚HCI, purchased from
Janssen Chimica, by using the neutralization method described
by Hunt.33 In a modified method of Wheeler,34 used to prepare
5B2HPM, the parent base 2HPM was dissolved in the solution
of 80 vol % acetic acid and 20 vol % acetic acid anhydride,
and bromine was added slowly during refluxation. The
precipitate was filtered and washed with acetic acid, and the
with an additional polarization function taken from Huzinaga
et al.,36 and this basis set is further referred to as the “BBS”
basis set. In all the calculations, the d-shells had six Cartesian
compounds. The IR frequencies and intensities and the zero-
point vibrational energy (ZPE) were computed at the SCF level
of theory using the analytical derivative procedure incorporated
in the Gaussian 92 program.37 Following these calculations,
the total energies of the tautomers were calculated at the
MP2+0.9ZPE level using the same basis set (the Møller-Plesset
second-order energy was calculated with the SCF/6-31++G**
optimal geometry; MP2/6-31++G**//SCF/6-31++G**). Cal-
culated frequencies were scaled down by the single factor 0.90
to approximately correct for vibrational anharmonicity, as well
as for the overestimation of the force constants at the SCF level.
compound was recrystallized from ethanol. The purity of both
synthesized compounds was checked with NMR, mass spec-
trometry, and IR spectroscopy in KBr dispersion, and they were
found to be >99% pure. Twice-distilled water was used for
the water-doped samples, while Ar gas (99.9949%) from Air
Liquide was used in all experiments.
Theoretical Method. Optimized geometries of the hydroxy
and oxo tautomers of 2HPM, 5F2HPM (5-fluoro-2-hydroxy-
pyrimidine), and 5C2HPM (5-chloro-2-hydroxypyrimidine) were
determined at the SCF/6-31++G** level of theory. In the case
of 5B2HPM, the standard 6-31++G** basis set was used for
H, C, N, and O atoms, while for the Br atom the double zeta
basis set from Ahlrichs35 with the contraction scheme (14s11p5d)/
[8s6p2d] was used. The Ahlrichs basis for Br was augmented

Molecules Modeling Cytosine or Isocytosine Tautomers
TABLE 2: Experimental (Ar Matrix) and Calculated (SCF/6-31++G**) IR Spectral Data for 2-Hydroxypyrimidine
experimental calculated
νb (cm-1 I (km‚mol-1
J. Phys. Chem. A, Vol. 102, No. 42, 1998 8159
ν (cm-1
)
Ia (km‚mol-1
)
)
)
PEDc
3591/3588
3075
3050
3015
1591
1573
1470/1467
1435/1433
1353
1333/1325
1210/1197
1168
1090
1080
1023
d
995
871
808
793
641
572
536
504/501
453
151f
1
2.5
3.5
112
3742
3062
3024
3019
1627
1614
1464
1450
1342
1324
1185
1139
1062
1043
1019
1006
987
870
818
794
632
584
534
493
448
151
5
22
ν(OH) (100)
ν(C5H) (92)
ν(C6H) (75) + ν(C4H) (23)
ν(C4H) (73) + ν(C6H) (20)
12
292
289
244
261
12
78
130
20
ν(C5C6) (21) + ν(C2N3) (17) + ν(C6N1) (19) + ν(N3C4) (13) + δ(C6H) (12)
ν(C4C5) (24) + ν(N1C2) (21) + ν(N3C4) (12) + δ(C4H) (11)
ν(C2O) (19) + δ(C4H) (15) + δ(C5H) (14) + ν(C2N3) (12)
δ(C5H) (21) + δ(C6H) (20) + ν(C2O) (12) + ν(N1C2) (11)
ν(C2O) (26) + δ(C4H) (22) + δ(C6H) (15) + ν(N3C4) (13) + ν(N1C6) (11) + δR1 (11)
δ(OH) (32) + δ(C4H) (30) + δ(C6H) (28)
105
111f
255f
86
142f
144f
60
δ(OH) (48) + δ(C6H) (12)
δ(C5H) (32) + ν(C6N1) (26) + ν(N3C4) (25) + ν(N1C2) (10) + ν(C2N3) (10)
ν(C5C6) (40) + ν(C4C5) (20) + δ(C6H) (12) + δ(C4H) (10)
ν(C4C5) (35) + δ(C5H) (21) + ν(N1C2) (19) + ν(C5C6) (15)
γ(C4H) (64) + γ(C6H) (43)
11
21
17
0.3
29
0
0.02
γ(C6H) (54) + γ(C4H) (33) + γ(C5H) (27)
5
42
69
11
17
3
52
193f
6
1
16
57
38
7
δR1 (54) + ν(C2N3) (10)
ν(C2O) (26) + ν(C2N3) (20) + δR1 (19) + ν(N1C2) (17)
γ(C2O) (55) + τR1 (48)
γ(C5H) (77)
δ
R2 (67) + δR3 (17)
1
δ
R3 (61) + δR2 (16) + ν(C2O) (14)
18
120
19
1
τ
R1 (38) + γ(OH) (26) + γ(C2O) (24) + τR2 (14)
γ(OH) (73) + γ(C2O) (11)
δ(C2O) (80)
405
e
30
415
199
τR3 (107)
R2 (87)
5
τ
a Experimental intensities normalized to the calculated value of n(OH) (151 km‚mol-1). b Uniform scaling factor 0.90. c Only contributions g10%
are listed; ν designates stretching, δ in-plane bending, γ out-of-plane deformation and τ torsion; subscript R ) ring mode. d Too weak to be
observed. e Situated below studied region (< 400 cm-1). f Total intensity of band pair (main band of pair in italics).
The same procedure was also applied to the H-bonded com-
plexes with water. In determining the H-bond energies, the basis
set superposition error (BSSE) was corrected by recalculating
the monomer energies in the basis set of the heterodimer
(counterpoise correction). Detailed arguments for the use of
the above approach have been described earlier.1
around 1730 cm-1 for monomeric nucleic bases and their
analogs.7,10-13,32,39 If the oxo tautomer is present in the sample,
the νCdO mode is usually very intense. Our assignment of the
weak bands in the 3500-3400 and 1770-1700 cm-1 regions
in the oxo tautomer is tentative. It is also possible that these
bands may be due to impurities in the sample. Provided that
our assignment is correct, we can use the combination of the
experimental and ab initio calculated intensities of the charac-
teristic νOH and νCdO modes to estimate the value of the
tautomerization constant KT(h/o),10-12 but due to the very small
experimental intensity of the νCdO band(s), only the order of
magnitude of its value can be obtained in the present case. Table
4 summarizes these experimental KT(h/o) estimations. Taking
into account the earlier reported value of (300 by Lapinski et
al.,6d which was obtained by taking into account only the main
Results and Discussion
2HPM and 5B2HPM. Table 1 summarizes the theoretical
results obtained for the hydroxy / oxo tautomerism in 2HPM
and 5B2HPM. For sake of comparison, calculations have also
been performed for the 5F and the 5Cl derivatives. The results
clearly demonstrate that the hydroxy form (Scheme 1, 5B2HPM)
is by far the most stable tautomer in each case, with the energy
difference to the less stable oxo form increasing with increasing
electronegativity of the halogen substituent at the ring C5
position. These results are consistent with ab initio and
experimental data for cytosines and 5-X-cytosines, which exhibit
the same trend: i.e., the hydroxy form is stablized by an
electron-withdrawing halogen atom at the ring C5 position.38
The (Ar) matrix IR spectrum of 2HPM has already been
described several times in the literature, but its spectrum was
analyzed by comparison with CNDO and lower level ab initio
(SCF/3-21G) results only.6d The experimental and theoretical
vibrational data for the hydroxy form obtained in this work are
summarized in Table 2. The theoretical results for the oxo
tautomer are summarized in Table 3. From the experimental
FT-IR spectrum (Figure 1), it is evident that the hydroxy
tautomeric form is highly predominant in Ar, in agreement with
the earlier investigations.6 This is manifested by the observation
of an intense, splitted νOH absorption at 3591 cm-1, while there
νCdO component of the experimental Fermi-resonance multiplet,
and a rather large estimated theoretical intensity ratio of the
CdO, and OH vibrations of 10.0, the value of (60 obtained
for 2HPM in this work is not in conflict with the earlier results.
We may therefore conclude that 2HPM occurs in the gas phase
and in matrices predominantly as the hydroxy tautomer, in
agreement with the ab initio data listed in Table 1. As far as
the assignment of the experimental spectrum of the 2HPM
monomer is concerned, except for a very weak γCH mode and
one ring torsion situated below the studied region, all funda-
mentals of 2HPM are rather easily assigned by comparison with
the theoretical data. The mean frequency deviation |νexp-νth|
(the νOH mode has been omitted because of its larger anhar-
monicity3) for the assignment is 13.6 cm-1, which is the usual
level of accuracy obtained for cytosine analogs using the SCF/
6-31++G** level of theory.1-5 However, the deviations
between experimental and predicted intensities are larger than
observed for most other cytosine-modeling molecules studied
so far.
are only some very weak bands in the νN-H (3500-3400 cm-1
)
and the νCdO region (1770-1700 cm-1). The latter feature is
characteristic for the oxo tautomer and is normally located

8160 J. Phys. Chem. A, Vol. 102, No. 42, 1998
Smets et al.
TABLE 3: Calculated (SCF/6-31++G**) IR Spectral Data for 2-Pyrimidone
calculated
νa (cm-1
)
I (km‚mol-1
)
PEDa
3484
3076
3048
2996
1762
1665
1563
1456
1410
1353
1178
1154
1088
1011
990
985
974
817
799
755
656
614
557
78
12
3
ν(NH) (100)
ν(C5H) (88) + ν(C6H) (11)
ν(C6H) (89) + ν(C5H) (11)
ν(C4H) (99)
27
1097
155
296
35
46
48
53
21
13
0.03
0.02
25
10
11
87
46
58
1
ν(C2O) (77)
ν(N3C4) (36) + ν(C5C6) (27)
ν(C5C6) (21) + ν(N3C4) (18) + δ(NH) (16) + ν(C4C5) (11)
ν(C6N1) (24) + δ(NH) (18) + δ(C6H) (15) + δ(C5H) (13) + ν(N3C4) (13)
δ(C4H) (23) δ(NH) (17) + δ(C6H) (17) + ν(C4C5) (13) + δ(C5H) (12)
δ(C4H) (41 + δ(NH) (17) + ν(N3C4) (14) + δ(C5H) (12)
δ(C6H) (39) + ν(C6N1) (23) + δ(NH) (14) + ν(C5C6) (11)
ν(N1C2) (29) + ν(C2N3) (28) + δ(C4H) (14) + δ(C5H) (11)
δ(C5H) (33) + ν(C6N1) (26) + ν(C5C6) (23)
γ(C4H) (84) + γ(C6H) (22)
γ(C6H) (72) + γ(C5H) (25) + γ(C4H) (17)
δR1 (72) + ν(C4C5) (11)
ν(C4C5) (55) + δR1 (18)
ν(C2N3) (45) + ν(N1C2) (30)
γ(C2O) (80) + τR1 (19)
γ(C5H) (76) + γ(C6H) (12)
γ(NH) (85) + τR1 (19)
δ
R2 (60) + δR3 (13)
R3 (66) + δR2 (19)
7
2
40
1
1
δ
483
473
382
134
δ(C2O) (68)
τ
τ
τ
R1 (45) + τR2 (24) + γ(NH) (18) + γ(C2O) (16)
R3 (106)
R2 (86) + τR1 (19)
a See Table 2, footnotes b and c.
Figure 1. FT-IR spectrum of 2-hydroxypyrimidine isolated in Ar at 12 K.
Figure 2 illustrates the FT-TR spectrum of 5BHPM in Ar.
Again it is obvious that the hydroxy tautomer is highly
predominant for this compound, since no νNH band is observed
(region 3500-3400 cm-1) and only a very weak band is found
in the νCdO region around 1732 cm-1. Multiplying the intensity
ratio of the strong νOH band (3590 cm-1) and the weak νCdO
band by the inverse ratio of the theoeretically predicted
intensities, an estimated KT(h/o) value of 184 is obtained (Table
4). As can be seen by comparing the data in Table 4 with the
data in Table 1, there is a dramatical difference between the
experimentally estimated KT(h/o) values and their predicted
values obtained from the approximate relation: -RT ln KT )
∆ET, where the energy difference between the oxo and hydroxy
tautomers, ∆ET, is taken from the theoretical calculations.
Interestingly, for 1-CH3-cytosine5 and 1-CH3-adenine,40 much
smaller differences between predicted and experimental KT
values were observed. We will comment on the unusually large
difference between the experimental and theoretical KT value
of 2HPM and 5BHPM in the Discussion Session.
Table 5 lists the experimental frequencies and intensities of
the predominant hydroxy tautomer 5B2HPM in comparison with
the ab initio predicted spectral parameters. Again, the assign-
ment can be performed for almost all the fundamentals, and
the mean frequency deviation is 14.3 cm-1 (νOH mode omitted).

Molecules Modeling Cytosine or Isocytosine Tautomers
J. Phys. Chem. A, Vol. 102, No. 42, 1998 8161
TABLE 4: Experimental Estimation of the Lower Limit
KT(h/o) Value for 2-Hydroxypyrimidine (2HPM) a
2-Pyrimidone (2OPM) and 5-Bromo-2-hydroxypyrimidine
(5B2HPM) a 5-Bromo-2-pyrimidone (5B2OPM)
water (Scheme 2) is the most stable 1:1 complex for both
systems (Table 6). The H bond energies calculated at the MP2/
6-31++G**//SCF/6-31++G** level of theory, corrected for
the BSSE, are 49 and 60 kJ/mol-1 for 2HPM and 2OPM,
respectively. These values manifest a strong H-bonding as-
sociation in both complexes. As a matter of fact, the H-bond
interaction energies for the N‚‚‚H-O‚‚‚HO complex of 2HPM
can be compared with the values obtained earlier at the same
level of theory for the singly H-bonded N‚‚‚HO-H pyrimidine
water complex and the O-H‚‚‚OH2 4-hydroxypyridine-water
band pair
νh (cm-1
)
νo (cm-1
)
A(νh)/A(νo) a(νo)/a(νh)a KT(h/o)
2HPM a 2OPM
ν
OH (3591)
OH (3591)
ν
ν
CO (1730)
CO (1730/1719/1713)
18.8
7.3
7.3
138
60
295
ν
8.2
literature6d
29.5b
10.0c
5B2HPM a 5B2OPM
complex, the interaction energies being 18 and 25 kJ/mol-1
,
νOH (3590)
ν
CO (1732)
26.3
7.0
184
respectively. The difference in the interaction energies of the
singly- and doubly H-bonded complexes indicates that H-bond
cooperativity definitely exists between the two H-bonds in the
closed 1:1 complex. This point is discussed further. Another
important result emerging from the ab initio calculations is that
the energy difference between the 2HPM and 2OPM tautomers
drops by half from 18 kJ/mol-1 in the free base (Table 1) to 9
kJ/mol-1 in the complex with water.
a Theoretical intensities taken from Tables 2 (2HPM) and 3 (2OPM)
or from Table 5 (5B2HPM), with aCdO (5B2OPM) ) 1209 km‚mol-1
.
b Only the main ν(CO) component has been taken into account.6d
c Crudely estimated theoretical intensity ratio.6d
Similar to the observation made for 2HPM, some rather large
deviations between experimental and predicted intensities are
noted.
The experimental FT-IR spectra of 2HPM and 5B2HPM
isolated in water-doped Ar matrices (Figures 3 and 4) exhibit a
series of important spectral effects induced by complexation
with water. For both compounds, an intensity decrease of the
bands assigned to the fundamental OH mode is observed
(especially after annealing). These modes include νOH at 3591
cm-1, δOH at 1333 and 1210 cm-1, and γOH at 504 cm-1 (2HPM/
H2O/Ar). On the other hand, the νCdO absorption bands, at
1713/1703/1679 cm-1 for 2HPM and at 1716/1665 cm-1 for
5B2HPM, which were very weak in the monomer spectrum,
show a clear intensity increase. This observation indicates that
the tautomeric equilibrium position is noticeably shifted due to
H2O doping of the matrix. A similar effect, although less
pronounced, was observed for 2-hydroxypyridine and for
4-hydroxypyrimidine.21 Apart from this important observation,
the appearance of the following bands due to the formation of
the water complex in the high-frequency region in the 2HPM
2HPM/H2O/Ar and 2B2HPM/H2O/Ar. In view of the close
similarity between 2HPM and 5B2HPM, the spectral results for
the two molecules will be presented together, while ab initio
results are only discussed for the nonsubstituted compound.
The hydroxy tautomer has three basic sites: the two
heterocyclic nitrogen atoms and the oxygen atom of the OH
group. On the other hand, the only proton-donor site available
is the hydrogen atom of the OH group. If the tautomeric
equilibrium is shifted from the hydroxy form toward the oxo
form in the presence of water, two additional H-bond interaction
sites of the oxo tautomer, i.e., the basic C2dO group and the
acidic N1-H group, should also be taken into account. The
proton affinity (PA) of 2-pyrimidone was estimated to be 870
kJ‚mol-1 41
Although ref 41 does not mention if this value
.
refers to the nitrogen atom or to the C2dO site, it was
demonstrated that both sites have very similar calculated PA
values in 2-pyridone.42
case should be mentioned: i.e., new νf(H O) bands at 3702-
2
3700 and 3693 cm-1, and new bands at 3463/3455, 3410, 3380,
The ab initio geometry optimizations performed for com-
plexes of 2HPM and 2OPM with a water molecule indicate that
the closed structure with two H bonds between the base and
3330, and 3260 cm-1. For 5B2HPM, similar new νfH O bands
2
at 3701 and 3694 cm-1, and new absorptions at 3463/3452,
Figure 2. FT-IR spectrum of 5-bromo-2-hydroxypyrimidine isolated in Ar at 12 K.

8162 J. Phys. Chem. A, Vol. 102, No. 42, 1998
TABLE 5: Experimental (Ar Matrix) and Calculated (SCF/BBS) IR Spectral Data for 5-Bromo-2-hydroxypyrimidine
experimental calculated
Ia (km‚mol-1 I (km‚mol-1
Smets et al.
ν (cm-1
)
)
νb (cm-1
)
)
PEDc
3590
3057
3050
172
5
3741
3041
3038
1625
1604
1463
1385
1347
1319
1181
1114
1063
1011
1005
973
871
813
648
642
525
489
459
415
172
4
5
ν(OH) (100)
ν(C6H) (99)
ν(C4H) (99)
1578/1574
1563
1445/1433
1386
1331
1311
1209/1203
1122
1139d
1014
1004
969
870
797
660
646
524
495
460
413
116f
93
358f
63
69
28
86f
17
50
10
6
261
212
628
49
22
78
141
16
42
20
0
ν(C6N1) (23) + ν(N3C4) (19) + ν(C5C6) (16) + ν(C2N3) (13)
ν(N1C2) (23) + ν(C4C5) (21) + ν(C2N3) (14)
ν(C2O) (30) + ν(N1C2) (17) + ν(C2N3) (13)
ν(N3C4) (23) + ν(C6N1) (18) + δ(C6H) (11) + ν(C4C5) (13)
ν(C2O) (28) + δ(C4H) (21) + ν(C6N1) (13) + ν(N3C4) (13) + δ(C6H) (13)
δ(C6H) (32) + δ(C4H) (34) + δ(OH) (28)
δ(OH) (48) + ν(C6N1) (19) + ν(N3C4) (14)
ν(C5C6) (32) + ν(C4C5) (25) + ν(C5Br) (12) + δ(C4H) (12)
ν(N1C2) (28) + ν(C2N3) (23) + ν(C4C5) (22) + ν(C5C6) (13)
δ
R1 (56) + ν(C2N3) (13)
γ(C4H) (59) + γ(C6H) (48)
3
6
6
1
γ(C6H) (57) + γ(C4H) (45)
ν(C2O) (23) + ν(C2N3) (20) + δR1 (17) + ν(N1C2) (17)
γ(C2O) (56) + τR1 (49)
24
14
13
18
77
17
8
54
11
13
23
118
20
1
1
0
2
0
δ
R2 (46) + ν(C5Br) (15) + ν(C2O) (12)
R3 (46) + δR2 (24)
δ
γ(OH) (34) + τR1 (33) + γ(C2O) (20)
γ(OH) (66) + γ(C2O) (13) + τR1 (12)
δ(C2O) (71)
τR2 (79) + τR3 (28)
e
e
e
e
297
294
210
104
γ(C5Br) (69) + τR1 (14)
ν(C5Br) (60) + δR3 (20)
δ(C5Br) (86)
τR3 (58) + τR2 (19) + γ(C5Br) (22)
a Experimental intensities normalized to the calculated value of ν(OH) (172 km‚mol-1). b,c See Table 2. d Tentative assignment. e,f See Table 2.
TABLE 6: Ab Initio 6-31++G** Calculated Energy Components (au), Total Energy and Basis Set Superposition Error
Corrected Interaction Energies (kJ‚mol-1) for Closed 1:1 H-Bonded Complexes of 2-Hydroxypyrimidine and 2-Pyrimidone with
H2Oa
method
2-hydroxypyrimidine‚‚‚water
2-pyrimidone‚‚‚water
SCF
MP2a
ZPEb
total
∆Ec
-413.6326378
-414.8611222
0.1034240
-414.7576982
0.00
-413.6304866
-414.8576779
0.1034172
-414.7542607
9.03
dipole moment (D)
4.54
5.17
SCF + ZPE
-413.5292138
-414.7576982
-337.5899888
-338.6132562
-76.0322399
-76.2349850
-51.40
-413.5270694
-414.7542607
-337.5834830
-338.6058765
-76.0320153
-76.2344931
-59.79
MP2 + ZPE
SCF (base with ghose water orbitals)
MP2 (base with ghost water orbitals)
SCF (water with ghost base orbitals)
MP2 (water with ghost base orbitals)
H-bond energy (SCF + ZPE)
H-bond energy (MP2 + ZPE)
H-bond energy (SCF/BSSE + ZPE)
H-bond energy (MP2/BSSE + ZPE)
-55.43
-64.18
-42.32
-48.81
-54.15
-60.24
a Calculations performed with molecular structures optimized at the SCF/6-31++G** level. b Only valence correlation is considered. c Calculated
as 0.9∑hνi. d Energy difference between isomeric complexes.
3391, 3380, 3320 and 3255 cm-1, are found. We will describe
the assignment of these bands separately.
pyridine, pyrimidine, and 1-CH3-2-oxopyrimidine at 3400, 3468,
and 3458/3449 cm-1, respectively.1,4 The frequency of this
mode in 2HPM is similar to its frequency in pyrimidine, but is
considerably larger than that in pyridine, which is consistent
with the smaller PA value of the pyrimidine nitrogen atom (PA
) 888 kJ/mol) compared to the pyridine nitrogen atom (PA )
924 kJ/mol).41 The νfOH absorption corresponding to the free
OH bond of H-bonded water in the open H-bonded 2HPM-
water complex is assigned to the band observed at 3702 cm-1
in samples with low water content. This is close to the
frequency of the corresponding mode in pyrimidine‚H2O (3703
cm-1).1
Hydroxy Tautomer. Since the hydroxy form is the most
abundant tautomer in the matrix experiment, it can be expected
that most of the new absorptions originate from formation of
H-bonded water complexes of this tautomer. Figures 3 and 4
illustrate the spectra of 2HPM/H2O and 5B2HPM/H2O under
different conditions. In the high-frequency region, the band
observed at 3463 cm-1 appears already at low H2O concentra-
tion. For 5B2HPM, the corresponding band is found at 3463
cm-1. This band of the complex can be reasonably assigned
to the νb mode of water in an open H-bonded N‚‚‚HO-H
OH
system because this intense mode, characteristic of water bonded
as a proton donor to a heterocyclic nitrogen atom, has been
observed before for the open N‚‚‚HO-H complexes with
In our former study on water complexes of 3- and 4-hy-
droxypyridine, we have observed bands characteristic of H
bonding between the base OH group acting as a proton donor

Molecules Modeling Cytosine or Isocytosine Tautomers
J. Phys. Chem. A, Vol. 102, No. 42, 1998 8163
SCHEME 2
the open N‚‚‚HO-H complex is observed in the experiment
and the open O-H‚‚‚OH2 complex is not.
The perturbed water νb and base νOH‚‚‚ modes of the ab
OH
initio predicted, closed complex N1‚‚‚HO‚‚‚H-O10 are both
expected to appear in the 3400-3300 cm-1 spectral region,
because this type of complex has stronger H bonds than the
corresponding open N‚‚‚H-OH and H-O‚‚‚H-O complexes.
This spectral region contains water polymer bands at 3373 and
3330 cm-1 for samples with larger water concentrations,44 and
their overlap with the closed-complex νbOH and νOH bands makes
assignment difficult. However, it is well-known that the
intensity of the polymer band at 3330 cm-1 is markedly smaller
than that of the band at 3373 cm-1 in H2O/Ar matrices.44 Figure
3 demonstrates that the reverse is true for 2HPM‚H2O and
5B2HPM‚H2O, and the larger intensity of the absorption at 3330
cm-1 suggests that the νOH‚‚‚ absorption in the closed N‚‚‚H-
O‚‚‚H-O structure overlaps with the polymer water band
located there. Furthermore, the other water polymer band at
3373 cm-1 exhibits a higher frequency shoulder at 3380 cm-1
,
for both 2HPM and 5B2HPM. Although this shoulder is also
present in H2O/Ar when a very high concentration of H2O is
used,44 this band is not expected to have such a large intensity
at the level of water doping used in the experiments. We
therefore assign the shoulder at 3380 cm-1 to the νb mode,
OH
and the band overlapping with the polymer water band at 3330
cm-1 to the base νOH‚‚‚ mode in the closed N1‚‚‚H-O‚‚‚H-
O10 structure. We have formerly assigned the νb mode for
OH
N‚‚‚H-O‚‚‚H-N closed complexes of 4-aminopyrimidine2 and
amino-oxo 1-methylcytosine5 with water to spectral features
at 3386 and 3350 cm-1, respectively. The corresponding νfOH
frequency for the closed complex should also be noticeably
smaller than the corresponding mode for the open N‚‚‚H-OH
complex, and this mode is therefore assigned to the bands found
at 3693 (2HPM) and 3694 (5B2HPM) cm-1
. apart from
intensity decreases of the monomer bands of the base OH group,
further evidence for H bonding of this group is obtained from
observation of shifted bendings and out-of-plane modes, e.g.,
for 2HPM at 1251 (δOH +41), 1375 (δOH +42), and 650 (γOH
+ 146) cm-1. These shifts are not larger than those observed
for similar modes in the 3- and 4-hydroxypyridine open O-H‚
‚‚O-H complex, and one could argue that this is not in
accordance with the expected larger shifts due to the cooperative,
closed H-bond interaction. One should keep in mind, however,
that the two H bonds in the closed complex are far from the
perfect “H bond” geometry. As a matter of fact, the N‚‚‚H-O
angle of the H bond in the closed complex of about 49° is much
larger than the similar angle in the open N‚‚‚H-OH complex
of pyrimidine which was predicted to be equal to 26° (in contrast
to 1.7° only in pyridine‚H2O).1
toward the oxygen atom of water (O-H‚‚‚OH2) complex.
Spectral manifestations of the complex were a shift of the base
νOH mode and, more interestingly, new and rather intense
The other H-bond angle O‚‚‚H-O is 164° in the closed
water-2HPM complex which is noticeably smaller than the
similar angle in the open O‚‚‚H-O complex of 4-OH-pyridine
equal to 178°. The distortion from the perfect alignment reduces
the H-bond cooperativity. Table 7 summarizes the experimental
and calculated frequencies for the closed complex of 2HPM.
Taking into account the increased anharmonicity of the bonded
absorptions in the νfOH water region at 3724 and 3721 cm-1 3
,
slightly below the absorption of the proton-acceptor unit in the
water dimer.43 Observation of new absorptions in this spectral
region in the case of 2HPM and 5B2HPM water complexes
could, therefore, be also taken as a manifestation of the presence
of open O-H‚‚‚OH2 H-bonded complexes. However, no such
absorptions were found. From the thermodynamical point of
view, as well as from the ab initio results, one expects that
formation of the energetically favored, closed water complexes
is more probable than of open complexes of either the O-H‚
‚‚OH2 or the N‚‚‚H-OH type. However, formation of the latter
type structure is only possible when the base OH bond is rotated
away from the water molecule and points toward the other N
atom of the pyrimidine ring. This is probably the reason why
OH νb and νOH‚‚‚ stretches, a good agreement between the
OH
experimental band shifts and the ab initio predicted shifts is
observed.
Oxo Tautomer. The rather strong intensity increase of the
νCdO absorption shown in Figure 4 undoubtedly indicates that
the abundance of the oxo tautomeric form increases when the
matrix doped with water and 2HPM or 5B2HPM is annealed.
This is in accordance with the ab initio predicted decrease of

8164 J. Phys. Chem. A, Vol. 102, No. 42, 1998
Smets et al.
Figure 3. High-frequency region of the FT-IR spectrum of 2-hydroxypyrimidine/H2O/Ar (left) and 5-bromo-2-hydroxypyrimidine/H2O/Ar (right):
(a) H2O/Ar ) 1/500; (b) annealed a; (c) H2O/Ar ) 1/100, annealed; (d) H2O/Ar ) 1/1000; (e) H2O/Ar ) 1/300; (f) H2O/Ar ) 1/200 annealed; v
) complex H2O modes; * ) shifted base modes; D,T,P ) water dimer, trimer, polymer bands.
Figure 4. Carbonyl stretching region of the FT-IR spectrum of 2-hydroxypyrimidine/H2O/Ar (left) and 5-bromo-2-hydroxypyrimidine/H2O/Ar
(right): (a, c, d, f) see Figure 3; * ) shifted base modes, *h ) appearance of νCdO mode(s); D,T,P ) water dimer, trimer, polymer bands.
the energy difference between the 2HPM and 2OPM tautomers
from 18 to 9 kJ/mol-1 when they are complexed with water.
The bands observed for the H-bonded complexes of the oxo
tautomer are predominantly those of the doubly H-bonded
complex of this tautomer (see Scheme 2h). Prominent complex
bands of 2OPM to be assigned are those observed at 3260, 1713/
We mentioned earlier the observation of the intense doublet
band at 3463/3455 cm-1 for both 2HPM‚H2O and 5B2HPM‚
H2O. The higher-frequency component of this doublet was
assigned to νbOH in the open N‚‚‚H-OH complex of the hydroxy
tautomer. The other component of the doublet is most probably
due to the νb mode in the closed CdO‚‚‚HsO‚‚‚HsN
OH
1703/1679, 1440, 826, and 760 cm-1
.
These bands can
complex. This latter mode is expected to exhibit a rather broad
absorption, as was the case for 2-pyridone‚H2O,13 and one may
wonder why, in the present case, the width of this mode is so
respectively be attributed to the νNH‚‚‚, νCdO‚‚‚ (with Fermi
components), δNH‚‚‚, γCdO‚‚‚, and γNH‚‚‚ modes of the complex.

Molecules Modeling Cytosine or Isocytosine Tautomers
J. Phys. Chem. A, Vol. 102, No. 42, 1998 8165
TABLE 7: Experimental (Ar Matrix) and Calculated (SCF/6-31++G**) IR Spectral Data for the Closed N1‚‚‚H-O‚‚‚H-O10
H-Bonded Complex of 2-Hydroxypyrimidine with Water
experimental
ν (cm-1 ∆ν (cm-1
calculated
a
b
a
)
)
ν (cm-1
)
I (km‚mol-1
)
∆ν (cm-1
)
optimal scalingc
PEDd
Water Modes
-28
3693
3380
1620
565?
-41
-299
+29
4239
4082
1743
578
187
222
154
89
0.870
0.828
0.929
νf(OH) (88) + νb(OH) (13)
νb(OH) (84) + νf(OH) (12)
δ(HOH) (96)
-64
+13
δ(‚‚‚OHH‚‚‚) (54)
Base Modes
-179
+8
3330
1476
1375
1251
810
-261
+6
3588
1471
1356
1226
821
478
297
125
83
67
186
0.833 (0.840)e
ν(OH) (96)
ν(C2O) (26) + δ(C4H) (11) + ν(C2N3) (11)
δ(OH) (47) + δ(C5H) (31)
δ(OH) (26) + δ(C6H) (20)
γ(C2O) (57) + τR (48)
γ(OH) (94)
+42
+41
+2
+31
+41
+3
650
+146
708
+215
a Shift with respect to experimental or calculated monomer frequencies (Table 2); calculated monomer water frequencies are 4267, 4146, and
1730 cm-1; for νbOH, the experimental shift is corrected for (reduced) coupling in bonded water.48 b Water modes unscaled, base modes uniformly
scaled with 0.90. c Optimal scaling factor obtained from νexp/νth. d Only contributions >10 are listed; description of intermolecular modes as in
4-NH2-pyrimidine‚H2O.2 e Value for the free base νOH mode.
TABLE 8: Experimental (Ar Matrix) and Calculated (SCF/6-31++G**) IR Spectral Data for the Closed
C2dO‚‚‚HsO‚‚‚HsN1 H-Bonded Complex of 2-Pyrimidone with Water
experimental
ν (cm-1 ∆ν (cm-1
calculated
a
b
a
)
)
ν (cm-1
)
I (km‚mol-1
)
∆ν (cm-1
)
optimal scalingc
PEDd
Water Modes
-29
3700
3455
1618
560?
-34
-217
+27
4238
4025
1739
600
157
224
425
196
0.873
0.858
0.930
νf(OH) (88) + νb(OH) (13)
νb(OH) (84) + νf(OH) (12)
δ(HOH) (96)
-121
+9
δ(‚‚‚OHH‚‚‚) (65)
Base Modes
-92
3260
1679
1440
826
e
3392
1736
1469
811
334
1080
90
141
17
0.865
ν(N1H) (99)
-34?
-26
ν(C2O) (65) + δ(N1H) (12)
δ(N1H) (24) + ν(C6N1) (16) + ν(N3C4) (14)
γ(C2O) (84) + γ(N1H) (27)
γ(N1H) (65) + τR (22) + γ(C5H) (14)
e
e
e
+13
+12
760
766
+110
a-d See Table 7. e Experimental shift not available because of very low abundancy of the free tautomer 2OPM.
small. Furthermore, for pyrimidine and 1-methyl-2-pyrimidone,
the νbOH bands of the open N‚‚‚H-OH complexes also exhibited
a doublet structure, which was explained by two different
orientations of the free OH bond of complexed water with
respect to the pyrimidine ring. The latter is rather unlikely in
the pyrimidine derivative with the hydroxy group adjacent to
the nitrogen atom, since the orientation of this group toward
the vicinal nitrogen atom should strongly favor formation of
the closed N‚‚‚H-O‚‚‚H-O complex. We therefore interpret
the doublet character of the bands at 3463/3455 cm-1 for 2HPM
and 5B2HPM as a combination of a relatively sharp νb
OH
absorption from the open N‚‚‚HO-H complex at 3463 cm-1
and a broader νb absorption from the closed CdO‚‚‚HsO‚
OH
‚‚HsN complex centered at 3455 cm-1. It will be argued in
Figure 5. FT-IR spectrum (2800-2400 cm-1) of 2-hydroxypyrimidine/
D2O/Ar. D2O/Ar 1/200, annealed; v ) complex absorptions: D,T,P )
dimer, trimer, polymer bands.
the Discussion section that observation of a free νCdO mode
after annealing the matrix is rather impossible in view of the
water-assisted tautomeric interconversion. The free νCdO mode
of 2OPM located around 1720 cm-1 is shifted by about 40 cm-1
in the complex and appears at 1679 cm-1. This shift is well
predicted by the ab initio calculations summarized in Table 8.
No bands characteristic to an open CdO‚‚‚HsOsH complex
are identified, and this can be understood in view of the origin
of the oxo tautomer as resulting from the proton-transfer reaction
converting the hydroxy to the oxo tautomer and operating in
the closed H-bonded complex (see further discussion). The
results shown in Tables 7 and 8 allow us to conclude that the
predicted shifts of the main modes of the two different closed
complexes agree rather well with the experimental findings.
2HPM/D2O/Ar. Additional arguments supporting the con-
clusions of the spectral analysis presented above was obtained
from isotopic D2O matrix-doping experiments. Figure 5 il-
lustrates the νOD frequency region (2800-2400 cm-1) of the
FT-IR spectrum of a 2HPM/D2O/Ar sample at relatively high
D2O concentration. The sharp bands observed between 2795
and 2580 cm-1 are all due to the D2O monomer (M), dimer
(D), and trimer (T) modes,44 except for a few bands due to the
HOD species. The latter are formed by isotopic exchange in
the glass vacuum line during preparation of the gaseous D2O/
Ar mixture and in the 2HPM/2OPM tautomeric interconversion.

8166 J. Phys. Chem. A, Vol. 102, No. 42, 1998
Smets et al.
Figure 7. Correlation between the optimal scaling factor for the bonded
water mode νbOH and the proton affinity value of the N(O) atom in 1:1
H-bonded complexes N(O)‚‚‚HO-H. 1 ) (H2O)2; 2-10 same as in
Figure 6.
Figure 6. Correlation between the (corrected) frequency shift of the
bonded water mode νb and the proton affinity value of the N atom
OH
in 1:1 H-bonded complexes N‚‚‚HO-H. 2 ) pyrimidine;1 3 ) 3-OH-
pyridine;3 4 ) benzimidazole;57 5 ) pyridine;1 6 ) imidazole;57 7 )
4-OH-pyridine;3 8 ) 4-NH2-pyridine;2 9 and 10 ) closed complexes
with 2-OH-pyrimidine (this work) and with 2-OH-pyridine,21 respec-
tively.
expected from the PAN value due to cooperativity between the
two H bonds in the closed complex, can be taken as a further
supporting argument for the assignment of the νbOH absorption
to the closed complex.
The 2570-2400 cm-1 region exhibits D2O-polymer (P)44 and
complex (D2O with 2HPM) absorptions, and the latter may be
correlated with the H2O analogs as indicated in the following
correlation scheme of νX-D//νX-H//ISR//vibrational mode data,
Optimal Scaling Factors for the νb
Mode in Closed
OH
H-Bonded Complexes. In the previous papers of this series,
we have demonstrated that the “optimal” scaling factor, νexp
/
νth (this factor, multiplied by the theoretical frequency, gives
where the isotopic ratio (ISR) is defined as νX-H/νX-D
2556/2547 // 3463/3455 // 1.355/1.356 // νN‚‚‚D(H)-O and
νCdO‚‚‚D(H)-O‚‚‚HN
:
the experimental frequency), for the bonded water mode νb
O
can be correlated with the proton affinity of the base atom, PAB,
which is H-bonded to water.1-5 The systematic decrease of
the optimal scaling factor with increasing PAB value originates
from increase of the anharmonicity of the H-bonded OH
stretching mode with increasing H-bond strength. Since the
ab initio frequency calculations in this work were performed
in the harmonic approximation, they have to be scaled to correct
for the anharmonicity. It can be anticipated that the relatively
strong closed H bonds should exhibit larger anharmonicity
leading to a smaller value of the scaling factor than in the case
of the open complexes with weaker interaction. This is indeed
what one observes in the correlation between the optimal scaling
factor and PAB as illustrated in Figure 7 for complexes of the
N‚‚‚HO(‚‚‚HO) type. This figure clearly shows the difference
between the closed and open complexes in their spectral
appearance. A manifestation of this in Figure 7 is that the point
representing the closed water H-bonded complex of 2HPM
deviates from the correlation curve for the N‚‚‚HO-H open
complexes. A similar observation was made before for the
closed H-bonded structure of 2-hydroxypyridine with water.21
Reduced Cooperativity between the H Bonds in Closed
H-Bonded Complexes. The H-bond cooperativity effect occurs
in systems where there are two or more H bonds and they
involve the same or closely neighboring proton-donor or proton-
acceptor sites.48 The cooperativity between the H bonds can
be either positive (H-bond strengthening) or negative (H-bond
weakening). The effect depends on whether the H-bond sites
and the interactions involved are of the same (donor-donor or
acceptor-acceptor) type or of the opposite (donor-acceptor)
type. In the case of the closed complexes of water with 2HPM
and 2OM, a positive cooperativity effect is expected since the
same water molecule acts simultaneously as a proton donor and
a proton acceptor, and the two H-bond interactions are of the
opposite type. A suitable method to evaluate H-bond cooper-
ativity in open linear trimers (HA)3, and higher multimers (HA)x,
is to compare the frequency shifts, ∆νOH for the bridged AH
bonds with the frequency shift of the AH bond stretch in the
open dimer, AH‚‚‚AH.49 Due to the experimental observation
2490 // 3380 // 1.357 // νN‚‚‚D(H)-O‚‚‚HO
2420 // 3260 // 1.347 // νN-D(H)‚‚‚
Furthermore, the relative intensities of the different complex
bands are largely similar to those in the spectrum of 2HPM/
H2O/Ar. The observation of the complex νN-D‚‚‚ mode is an
additional, important proof of the D2O-assisted tautomeric
interconversion of the N‚‚‚D-O‚‚‚H-O-C complex to the
N-D‚‚‚O-H‚‚‚OdC complex of the oxo tautomer.
Discussion
Open and Closed H-Bonded Base‚H2O Complexes in the
Vibration Correlation Diagram. Pimentel and Ault introduced
the so-called vibration corrrelation diagram for H-bonded and
proton-transfer base‚HX complexes by considering the fre-
quency shifts of the HX stretching mode as a function of the
difference of the proton affinity (∆PA) value for the base and
its acid anion.45 This concept has been applied by some of us
to a series of H-bonded complexes of different bases with
water.46 The vibration correlation diagram involves the cor-
relation between the frequency shift of the bonded-water
stretching vibration (νbOH) and the PAB value of the base.
Different correlations were observed for different H-bond centers
(π, O, N). The difference originates from a different degree of
the charge transfer for different basic atoms and different sizes
of their lone pairs.47 If the corrected frequency shift ∆νbOH of
H-bonded water for a series of matrix-isolated complexes, N‚
‚‚H-OH or CdO‚‚‚HsOH, is plotted as a function of PAN
and PACdO the points representing the closed H-bonded systems,
N‚‚‚H-O‚‚‚HO and CdO‚‚‚HsO‚‚‚HsN, exhibit rather large
deviation from the correlation curve for the open N‚‚‚H-OH
and CdO‚‚‚HsOH complexes. This is illustrated in Figure 6.
The deviation, which is caused by a larger frequency shift than

Molecules Modeling Cytosine or Isocytosine Tautomers
J. Phys. Chem. A, Vol. 102, No. 42, 1998 8167
of the νbOH modes for the open N‚‚‚HO-H complex of 2HPM,
as well as for the closed N‚‚‚HO‚‚‚HO complexes, it was
possible to evaluate the cooperativity factor, Ab, for the bridged
OH bond in the closed complex from the ∆νbOH(N‚‚‚H-O‚‚‚
H-O)/∆νbOH(N‚‚‚HO-H) ratio. The value of 302/207 ) 1.46
was obtained. This value indicates a substantial cooperativity
effect, since for linear 1:2 B‚‚‚H-O‚‚‚HO water complexes the
average ratio was 1.37.49 On the other hand, for the cyclic water
trimer (H2O)3, a much larger value of 1.73 was found,49 which
was explained by a mutual fortification of the extra H bond
which closes the system to form the cyclic trimer. In the closed
H-bonded systems such as those studied in this work, geo-
metrical constraints reduce the H-bond cooperativity. The
situation is different when the two H bonds occur in a more
ideal, “almost linear” configuration. We illustrated this point
in our previous studies where we compared the closed N‚‚‚H-
O‚‚‚HO complex of 2HPM‚H2O with the open N‚‚‚HO-H and
HO‚‚‚H-O water complexes of 4-NH2-pyridine3 and 4-OH-
pyridine,3 respectively.
either from the IR (Globar) source or from the interferometer
laser. It is therefore unlikely that an electronic excitation is
involved in the process.
A possible explanation of a double proton transfer in the
tautomerization process of the 2HPM/2OPM/H2O system can
be proton tunneling. Recently, Hammes-Schiffer and Tully have
demonstrated using the molecular dynamics quantum transition
(MDQT) method that proton transfer in the phenol‚amine
complex in methyl chloride solution is enhanced in vibrationally
excited levels.24,56 Since the MDQT method treats H and D
atoms quantum mechanically, the proton-transfer reaction can
occur below the barrier through quantum tunneling. Further-
more, it is a faster process than the vibrational relaxation.24,51
Such a tunneling may take place in the 2HPM/2OPM water-
assisted tautomeric interconversion. There are other examples
of proton tunneling assisted by solvent. A ground-state reverse
proton transfer (GSRPT) was found in 7-hydroxyquinoline
catalyzed by methanol in studies of Chou and Martinez.52,53
These experiments provided support for a two-step mechanism
of the GSRPT in 7-hydroxyquinoline proposed by Konijnenberg
et al.;54 the first step involving thermally-induced fluctuations
into a well-defined conformation suitable for the proton-transfer
process and the second step involving the tunneling. The
mechanism can be similar in the proton-transfer process
observed in the present work for 2HPM and 5B2HPM. Since
the GSRPT process in 7-hydroxyquinoline occurs at room
temperature and the first solvation shell around the solute
molecule involves two solvent molecules, the process is
accompanied by a large entropy decrease and a rather small
decrease in enthalpy resulting in a large, positive free enthalpy
change.52 In a low-temperature matrix, the structural reorga-
nization is probably not as significant in terms of free enthalpy,
since the low-temperature favors the enthalpy-decreasing forma-
tion of the closed complex, and the effect of the negative entropy
change is strongly reduced.55 However, it may still be sufficient
to drive the reaction forward.
A Double Proton-Transfer Mechanism in the Ground
State. It is clear from the spectral results presented above that
the tautomeric equilibrium of the 2HPM/2OPM system is shifted
toward the oxo form in the presence of water, but the effect is
only clear after the matrix is annealed. This indicates that water
assists in the tautomeric interconversion and that this intercon-
version takes place below 40 K which is the upper limit of the
annealing temperature. The process of matrix annealing, which
is usually performed for about 15 min, very likely produces
the most stable closed 2HPM‚H2O complex. Though, as we
mentioned before, the water-to-base ratio of 1:1 to 5:1 was
used which should ensure formation of an excess amount of
the 1:1 complex, some amounts of complexes with a higher
water content could also be formed in the annealing process.
In this complex, the protomeric reaction takes place. There have
been several theoretical and experimental studies, which we will
now review, that provide some explanation regarding the
possible mechanism of this reaction.
Summary
The first piece of evidence is provided by ab initio calcula-
tions of the barrier height for the protomeric reaction with and
without a water molecule. Several authors reported ab initio
calculated barrier heights for proton transfer in the isolated
tautomers of 2-hydroxypyridine and these vary between 190
and 140 kJ/mol.15,20 The barrier height decreases drastically
to about 50 kJ/mol when water is H-bonded to 2-hydroxypy-
ridine, allowing for a concerted mechanism with a double proton
jump to operate.20 A similar barrier height of 48.2 kJ/mol (MP2/
DZ95** result) was found in ab initio calculations for the
intermolecular double proton transfer process of 2HPM com-
plexed with one water molecule.26,27 It is possible that if more
water molecules are involved, the barrier lowers even more.
The barrier heights for proton transfer between the isolated
hydroxy and oxo tautomers indicate that proton movement
cannot effectively occur at 40 K. However, the lowering of
the barrier by water may enable the transfer.
The FT-IR matrix-isolation technique and ab-initio theoretical
calculations have been used to study the H-bonding interaction
of water with 2-hydroxypyrimidine and its 5-bromo derivative.
Both compounds occur predominantly in the hydroxy tautomeric
forms in Ar matrices, and the experimentally estimated values
for the hydroxy/oxo equilibrium constants, KT(h/o), are 60 and
184, respectively. Interpretation of the IR spectra of the
complexes with water assisted by ab initio calculated energies,
frequencies, and intensities indicates the presence of the closed
H-bonded N‚‚‚H-O‚‚‚H-O complex of the hydroxy tautomer
in the early stages of the experiment, but matrix annealing
induces an increasing presence of the closed CdO‚‚‚HsO‚‚‚
HsN complex of the oxo tautomer. An ab initio assisted
assignment of the main bands of the two complexes is presented.
We demonstrated with the use of the earlier established relations
that a relatively strong cooperativity exists between the two H
bonds in the closed complexes. An analysis of the character-
istics of the experimentally observed hydroxy f oxo tautom-
erism in water-doped matrices suggests that the mechanism of
the proton transfer involves proton tunneling. However, on the
basis of the results of the present study, we could not definitely
determine whether the proton transfer takes place in the 1:1
closed 2HPM‚H2O complex or it involves complexes with
higher water content.
Secondly, ab initio calculations on the lowest excited states
of the 2HPM-water complexes26 indicated that a possible route
for the tautomeric interconversion may involve electronic
excitation of the complex. The barrier height of only 18.3 kJ/
mol was found in CASPT2/DZ95** calculations of the first
electronically excited state, S1 (1ππ*). The calculations also
predicted that the reaction should be strongly exothermic by
about 64.6 kJ/mol. In the experiment performed in this work,
the matrix sample remained inside the cryostat positioned in
the FT-IR instrument, so that the sample can only absorb energy
Acknowledgment. This work was part of the ECC research
project ERB CIPA-CT93-0108, and cooperation between the

8168 J. Phys. Chem. A, Vol. 102, No. 42, 1998
Smets et al.
(21) Smets, J.; Vrancken, H.; Adamowicz, L.; Maes, G., manuscript in
preparation.
groups of Leuven and Tucson was supported by the NATO
International Collaborative Grant INT-9313268. G.M. acknowl-
edges the Belgian NFWO for a permanent Research Fellowship.
J.S. acknowledges the support of the Belgian IWONL. L.A. and
J.S. acknowledge the support from the Office of Health and
Environmental Research, Office of Energy Research and the
Department of Energy (Grant No. DEFG0393ER61605).
(22) Vener, M. V.; Scheiner, S. J. Phys. Chem. 1995, 99, 642.
(23) Luth, K.; Scheiner, S. J. Phys. Chem. 1995, 99, 7352.
(24) Hammes-Schiffer, S.; Tully, J. C. J. Phys. Chem. 1995, 99, 5793.
(25) Kasha, M.; Heldt, J.; Gormin, D. J. Phys. Chem. 1995, 99, 7281.
(26) Sobolewski, A. L.; Domcke, W. In The Reaction Path in Chem-
istry: Current Approaches and PerspectiVes, Ab-initio studies of Reaction
Paths in Excited-State Hydrogen-Transfer Processes: Mezey, P. G., Ed.;
Kluwer Academic Publishers: New York, 1995; pp 257-282.
(27) Sobolewski, A. L.; Adamowicz, L. J. Phys. Chem. 1995, 99, 14277.
(28) Forberg, S.; Solbakk, J. Acta Chem. Scand. 1970, 24, 3230.
(29) Beak, P.; Fry, F. S.; Lee, J.; Steele, F. J. Am. Chem. Soc. 1976, 98,
171.
(30) Person, W. B.; Szczepaniak, K.; Szczesniak, M.; Kwiatkowski, J.
S.; Hernandez, L.; Czerminski, R. J. Mol. Struct. 1989, 194, 239.
(31) Maes, G. Bull. Soc. Chim. Belg. 1981, 90, 1093.
(32) Graindourze, M.; Smets, J.; Zeegers-Huyskens, Th.; Maes, G. J.
Mol. Struct. 1990, 222, 345.
(33) Hunt, R. R.; McOmie, J. F.; Sayer, E. R. J. Chem. Soc. 1959, 525.
(34) Wheeler, K. J. Am. Chem. Soc. 1909, 42, 301.
(35) Schafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
(36) Gaussian Basis Sets for Molecular Calculations; Huzinaga, S., Ed.;
Elsevier: New York, 1984.
(37) Gaussian 92; Frisch, C. P. M. J.; Trucks, G. W.; Head-Gordon,
M.; Gil, P. M. W.; Wong, W. M.; Foresman, J. B.; Johnson, B. G.; Schlegel,
H. B.; Robb, M. A.; Replogie, E. S.; Gomperts, R.; Andres, J. L.;Ragha-
vachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. L.; Fox, D. J.; Defrees,
D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian Inc.: Pittsburgh,
PA, 1992.
Supporting Information Available: Table of predicted
geometry parameters of 2-hydroxypyrimidine and its N1‚‚‚H-
O‚‚‚H-O10 H-bonded complex with water, and of 2-pyrimidone
and its C2dO‚‚‚HsO‚‚‚HsN1 H-bonded complex with water
(1 page). Ordering information is given on any current mast-
head page.
References and Notes
(1) Destexhe, A.; Smets, J.; Adamowicz, L.; Maes, G. J. Phys. Chem.
1994, 98, 1506.
(2) Smets, J.; Adamowicz, L.; Maes, G. J. Phys. Chem. 1995, 99, 6387.
(3) Buyl, F.; Smets, J.; Maes, G.; Adamowicz, L. J. Phys. Chem. 1995,
99, 14967.
(4) Smets, J.; Destexhe, A.; Adamowicz, L.; Maes, G. J. Phys. Chem.
1997, 101, 6583.
(5) Smets, J.; Maes, G.; Adamowicz, L. J. Phys. Chem. 1996, 101,
6434.
(6) Nowak, M. J.; Szczepaniak, K.; Barski, A.; Shugar, D. J. Mol.
Struct. 1980, 62, 47. Czerminski, R.; Kuczera, K.; Rostkowska, H.; Nowak,
M. J.; Szczepaniak, K. J. Mol. Struct. 1986, 140, 235. Radchenko, E. D.;
Plokhotnichenko, A. M.; Sheina, G. G.; Blagoi, Yu. P. Biofiziku 1989, 28,
559. Lapinski, L.; Czerminski, R.; Nowak, M. J.; Fulara, J. J. Mol. Struct.
1990, 220, 147.
(7) Smets, J.; Maes, G. Chem. Phys. Lett. 1991, 187, 532.
(8) Nowak, M. J.; Lapinski, L.; Fulara, J.; Les, A.; Adamowicz, L. J.
Phys. Chem. 1992, 96, 1562.
(38) Jaworski, A.; Szczepaniak, M.; Szczepaniak, K.; Kubulat, K.;
Person, W. B. J. Mol. Struct. 1990, 223, 63.
(39) Person, W. B.; Szczepaniak, K. Vibrational Spectra and Structure;
Durig, J. R., Ed.; Elsevier: Amsterdam, 1993.
(40) Schoone, K.; Houben, L.; Smets, J.; Adamowicz, L.; Maes, G.
Spectrochim. Acta A, in press.
(41) Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data
1984, 13, 695.
(9) Lapinski, L.; Fulara, J.; Nowak, M. J. Spectrochim. Acta 1990, 46A,
61.
(10) Szczesniak, M.; Kwiatkowski, J. S.; KuBulat, K.; Szczepaniak, K.;
Person, W. B. J. Am. Chem. Soc. 1988, 110, 8319.
(11) Lapinski, L.; Nowak, M. J.; Fulara, J.; Les, A.; Adamowicz, L. J.
Phys. Chem. 1990, 94, 6555.
(12) Vrancken, H.; Smets, J.; Maes, G.; Lapinski, L.; Nowak, M. J.;
Adamowicz, L. Spectrochim. Acta A 1994, 50, 875.
(13) Smets, J.; Maes, G.’ Adamowicz, L., manuscript in preparation.
(14) Beak, P.; Covington, J. B.; Smith, S. G.; White, J. M.; Ziegler, J.
M. J. Org. Chem. 1980, 45, 1354.
(15) Field, M. J.; Hillier, J. H. J. Chem. Soc., Perkin Trans. 2 1987,
617.
(42) Katritzky, A. R.; Szafran, M.; Stevens, J. J. Mol. Structure
(THEOCHEM) 1989, 184, 179.
(43) Engdahl, A.; Nelander, B. J. Mol. Struct. 1989, 193, 101.
(44) Bentwood, R. M.; Barnes, A. J.; Orville-Thomas, W. J. J. Mol.
Spectrosc. 1980, 84, 381.
(45) Ault, B. S.; Pimentel, G. C. J. Phys. Chem. 1975, 79, 615.
(46) Maes, G.; Smets, J. J. Mol. Struct. 1992, 270, 141.
(47) Allen, L. C. J. Am. Chem. Soc. 1975, 97, 6921.
(48) Huyskens, P. L. J. Am. Chem. Soc. 1977, 99, 2578.
(49) Maes, G.; Smets, J. J. Phys. Chem. 1993, 97, 1818.
(50) Les´, A.; Adamowicz, L.; Nowak, M. J.; Lapinski, L. J. Mol. Struct
(THEOCHEM) 1992, 277, 313.
(51) Hammes-Schiffer, S.; Tully, J. C. J. Chem. Phys. 1994, 101, 4567.
(52) Chou, P.; Martinez, S. S. Chem. Phys. Lett. 1995, 235, 463.
(53) Lavin, A.; Collins, S. Chem. Phys. Lett. 1993, 207, 514.
(54) Konijnenberg, J.; Ekelmans, G. B.; Huizer, A. H.; Varma, A. G.
O. J. Chem. Soc., Faraday Trans. II 1989, 85, 39.
(16) Held, A.; Pratt, D. W. J. Am. Chem. Soc. 1993, 115, 9708.
(17) Favero, S.; Hatherley, L. D.; Brown, R. D.; Goddfrey, P. D.; Pierlot,
A. P.; Canlinatti, W.; Damiani, D.; Melandri, B. J. Phys. Chem. 1993, 97,
46.
(18) Szafran, M.; Karelson, M. M.; Katritzky, A. R.; Koput, J.; Zerner,
M. C. J. Comput. Chem. 1993, 14, 371.
(19) Del Belie, J. E. J. Phys. Chem. 1994, 98, 5902.
(20) Barone, V.; Adamo, C. J. Phys. Chem. 1995, 99, 15062.
(55) Maes, G. In Low Temperature Molecular Spectroscopy; NATO ASI;
Fausto, R., Ed.; Kluwer Academic Press: New York, in press.
(56) Maes, G.; Smets, J.; Adamowicz, L.; McCarthy, W.; Van Bael,
M. K.; Houben, L.; Schoone, K. J. Mol. Struct. 1997, 315, 410.