Tautomer selective photochemistry in 1-(tetrazol-5-yl)ethanol

Article abstract of DOI:10.1021/jp109215q

A combined matrix isolation FTIR and theoretical DFT/B3LYP/6-311++G(d,p) study of the molecular structure and photochemistry of 1-(tetrazol-5-yl)ethanol [1-TE] was performed. The potential energy surface landscapes of the 1H and 2H tautomers of the compound were investigated and the theoretical results were used to help characterize the conformational mixture existing in equilibrium in the gas phase prior to deposition of the matrices, as well as the conformers trapped in the latter. In the gas phase, at room temperature, the compound exists as a mixture of 12 conformers (five of the 1H tautomer and seven of the 2H tautomer). Upon deposition of the compound in an argon matrix at 10 K, only three main forms survive, because the low barriers for conformational isomerization allow extensive conformational cooling during deposition. Deposition of the matrix at 30 K led to further simplification of the conformational mixture with only one conformer of each tautomer of 1-TE surviving. These conformers correspond to the most stable forms of each tautomer, which bear different types of intramolecular H-bonds: 1H-I has an NH···O hydrogen bond, whereas 2H-I has an OH···N hydrogen bond. Upon irradiating with UV light (λ > 200 nm), a matrix containing both 1H-I and 2H-I forms, an unprecedented tautomer selective photochemistry was observed, with the 2H tautomeric form undergoing unimolecular decomposition to azide + hydroxypropanenitrile and the 1H-tautomer being photostable.

Full text of DOI:10.1021/jp109215q

1
3076
J. Phys. Chem. A 2010, 114, 13076–13085
Tautomer Selective Photochemistry in 1-(Tetrazol-5-yl)ethanol
†
†
,‡
‡,§
A. Ismael, M. L. S. Cristiano, R. Fausto,* and A. G o´ mez-Zavaglia
CCMAR and Department of Chemistry and Pharmacy, F.C.T., UniVersity of AlgarVe, Campus de Gambelas,
8
005-039 Faro, Portugal, Department of Chemistry, UniVersity of Coimbra, Coimbra, P-3004-535, Portugal,
and Centro de InVestigaci o´ n y Desarrollo en Criotecnolog ´ı a de Alimentos (Conicet La Plata, UNLP)
RA-1900, Argentina
ReceiVed: September 27, 2010; ReVised Manuscript ReceiVed: NoVember 9, 2010
A combined matrix isolation FTIR and theoretical DFT/B3LYP/6-311++G(d,p) study of the molecular structure
and photochemistry of 1-(tetrazol-5-yl)ethanol [1-TE] was performed. The potential energy surface landscapes
of the 1H and 2H tautomers of the compound were investigated and the theoretical results were used to help
characterize the conformational mixture existing in equilibrium in the gas phase prior to deposition of the
matrices, as well as the conformers trapped in the latter. In the gas phase, at room temperature, the compound
exists as a mixture of 12 conformers (five of the 1H tautomer and seven of the 2H tautomer). Upon deposition
of the compound in an argon matrix at 10 K, only three main forms survive, because the low barriers for
conformational isomerization allow extensive conformational cooling during deposition. Deposition of the
matrix at 30 K led to further simplification of the conformational mixture with only one conformer of each
tautomer of 1-TE surviving. These conformers correspond to the most stable forms of each tautomer, which
bear different types of intramolecular H-bonds: 1H-I has an NH · · · O hydrogen bond, whereas 2H-I has an
OH · · ·N hydrogen bond. Upon irradiating with UV light (λ > 200 nm), a matrix containing both 1H-I and
2
H-I forms, an unprecedented tautomer selective photochemistry was observed, with the 2H tautomeric form
undergoing unimolecular decomposition to azide + hydroxypropanenitrile and the 1H-tautomer being
photostable.
Introduction
Much attention has been paid to tetrazole (CN
confined. Because molecular diffusion in the solid low temper-
ature matrices is in general inhibited, no secondary reactions
usually take place involving species resulting from different
precursor molecules. Such simplification is very useful for
mechanistic elucidations.
4
2
H ) and its
derivatives due to their practical importance for medicine, agri-
culture, petroleum industry, photography, and chemical synthesis.1
-20
From a fundamental point of view, tetrazoles are also particularly
interesting systems, especially because of the tautomerism they may
1
-(Tetrazol-5-yl)-ethanol (C
3 7 4
H N O; 1-TE) has been used in
1,21-29
the design and synthesis of tetrazole-saccharinate conjugates,
exhibit and their diversified photochemistry.
39
which receive important uses as multidentate nitrogen ligands.
Unsubstituted tetrazole, for example, was found to exist
exclusively in the crystalline phases as the 1H-tautomer,3
0-32
The compound has the important structural characteristic of
being able to establish different types of intramolecular hydrogen
bonds (NH · · ·O or OH· · ·N), depending on the tautomeric form.
In turn, these different types of intramolecular interactions can
be expected to influence the chemical behavior of the tautomers.
In the present study, we performed a detailed systematic study
of the conformational space of the two tautomeric forms of 1-TE
and investigated their photochemistries. As described in detail
below, an unprecedent tautomer selective photochemistry was
observed, with the 2H-tautomeric form undergoing prompt
unimolecular decomposition when submitted to UV (λ > 200
nm) irradiation under matrix-isolation conditions, whereas the
1H-tautomer was found to be photostable. Matrix isolation
infrared spectroscopy was used as the main experimental
technique, with the experimental results receiving support from
extensive quantum chemistry calculations.
whereas in solution and gas phase 1H- and 2H-tautomers
coexist, the population of the most polar 1H form increasing
2
1,33-37
with the polarity of the solvent.
According to the
experimental findings, 2H-tetrazole was found to be the lowest
energy tautomer in media with a dielectric constant smaller than
about 7, while in media with a larger dielectric constant, a
reversal of energy takes place and 1H-tetrazole becomes the
2
1,38
tautomeric ground state.
The presence of both 1H- and 2H-
tautomers of substituted tetrazoles in the gas phase has also been
unequivocally proven in a series of recent studies we undertook
using matrix isolation infrared spectroscopy as the main
2
1,28
experimental technique.
When used together with infrared
spectroscopy, matrix isolation is a powerful technique for
undertaking structural studies. Indeed, for a matrix-isolated
compound, it is possible to achieve an essential simplification
of the spectra and high spectroscopic resolution, allowing to
distinguish spectral signatures of different tautomers or con-
formers. In addition, for matrix-isolated species, unimolecular
photochemistry is expected, with the products being cage-
Experimental and Computational Methods
Synthesis of 1-(Tetrazol-5-yl)ethanol. The synthetic route
to 1-TE involved construction of the heterocycle, which was
achieved by adapting the methodology developed by Sharpless
*
To whom correspondence should be addressed. E-mail: rfausto@ci.uc.pt.
University of Algarve.
University of Coimbra.
40,41
and Demko.
Sodium azide (1.43 g; 22 mmol), zinc bromide
†
‡
§
(4.50 g; 20 mmol), 2-hydroxy-propanenitrile (1.45 mL; 20
mmol), and water (50 mL) were mixed in a 250 mL round-
Conicet La Plata, UNLP.
1
0.1021/jp109215q  2010 American Chemical Society
Published on Web 11/29/2010

Photochemistry in 1-(Tetrazol-5-yl)ethanol
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13077
bottomed flask, and the mixture was stirred under reflux for
2
0 h. After cooling, hydrochloric acid (3 N, 30 mL) and ethyl
acetate (100 mL) were added, and vigorous stirring was
continued until no solid was present and the aqueous layer
attained a pH of 1. The organic layer was separated and the
aqueous layer extracted with ethyl acetate (2 × 50 mL). The
combined organic layers were evaporated, aqueous NaOH (0.25
M; 200 mL) was added to the residue, and the mixture was
stirred for 30 min until the original precipitate was dissolved
and a suspension of zinc hydroxide was formed. The suspension
was filtered, and the solid was washed with aqueous NaOH (1
M; 20 mL). Hydrochloric acid (3 N, 40 mL) was added to the
filtrate with vigorous stirring causing the separation of an oil.
After addition of a saturated solution of NaCl (brine), the
aqueous layer was extracted with ethyl acetate (3 × 100 mL).
The combined organic layers were dried over anhydrous sodium
sulfate and evaporated to afford the final product as a white
amorphous powder (1.70 g; 75% yield). IR (room temperature;
Figure 1. Most stable conformers of the two tautomeric forms of
1
-(tetrazol-5-yl)ethanol and adopted atom numbering scheme: (a) 1H-
tautomer; (b) 2H-tautomer. The dashed lines represent intramolecular
hydrogen bonds. In the 1H form, N C C O and C C O H are 176.2
4
5
6
8
5
6
8
14
and 178.9 degrees, respectively; in 2H, these angles are -35.2 and
-
1
4
2.2 degrees. ∆E(1H-2H) ) 0.75 kJ mol .
-
1 1
KBr pellet) νmax: 3390 (OH), 1696, 1624, 1243, 1123 cm ; H
NMR (CDCl
0
TABLE 1: Zero Point Corrected Relative Energies (∆E /kJ
): δ 7.55 (br, 1H), 7.22-7.35 (q, 1H), 7.35 (d,
-1
3
mol ) Obtained at the DFT(B3LYP)/6-311++G(d,p) Level
of Theory and Predicted Populations (p/%) at Room
Temperature (RT ) 25 °C) and 50 °C of the Various
+
3
+
1
H); MS (EI): m/z 115 (33%; [M + H] ), m/z 132 (100%; [M
+
4 3 7 4
NH ] ). Acc. Mass (CI): calcd for C H N O, 115.1134; found,
a
15.1145.
Infrared Spectroscopy and Photochemical Experiments.
The infrared (IR) spectra of 1-TE were obtained using a Mattson
Infinity 60AR Series) Fourier transform infrared spectrometer,
Conformers of 1-TE
tautomer
conformer
∆E0
p (RT)
p (50 °C)
1
H
I
II
0.75
3.94
17.8
4.9
17.4
5.3
(
equipped with a deuterated triglycine sulfate (DTGS) detector
and a Ge/KBr beam splitter, with 0.5 cm- spectral resolution.
The compound was placed in a pyrex L-shaped tube connected
to a needle valve, and both sample compartment and valve
nozzle were kept at about 50 °C during deposition of the
matrices. Matrices were prepared by codeposition onto the
cooled (10, 30 K) CsI substrate of the cryostat (APD Cryogenics
closed-cycle helium refrigeration system with a DE-202A
expander) of 1-TE vapors and a large excess of the matrix gas
III
IV
V
4.36
9.50
10.12
4.2
0.5
0.4
27.9
24.1
21.4
11.6
9.4
4.3
0.7
0.7
72.1
4.6
0.7
0.5
28.5
23.1
20.6
11.7
9.6
4.7
0.9
0.8
71.5
1
(
I
all 1H)
2
H
0.00
0.30
1.82
2.35
4.28
8.73
8.87
II
III
IV
V
VI
VII
(
argon N60, Air Liquide) coming from a separate line.
(
all 2H)
Irradiation of the samples was carried out with a 150 W xenon
a
The conformers are depicted in Figures S1 and S2. Geometrical
parameters are provided in Tables S1 and S2. The calculated total
zero point corrected energy of the most stable conformer is
1082302.79 kJ mol .
arc lamp (Osram XBO 150 W/CR OFR) through a quartz
window (λ > 200 nm).
Computational Methods. The quantum chemical calculations
were performed at the DFT level of theory using the standard
-1
-
4
2
6
-311++G(d,p) basis set and the three-parameter density
functional abbreviated as B3LYP, which includes Becke’s
Results and Discussion
4
3
gradient exchange correction and the Lee, Yang, Parr cor-
relation functional.44 Geometrical parameters were optimized
using the Geometry Direct Inversion of the Invariant Subspace
Geometries and Energies: The Potential Energy Land-
scape of 1-TE. 1-TE has a chiral center, the two enantiomers
R and S forms) being spectroscopically equivalent. In the
present study we will focus on the R form. The compound may
exist in two tautomeric forms (Figure 1), both having two
conformationally relevant internal degrees of rotation, which
(
45
(
GDIIS) method. Optimization of geometries was followed
by calculation of the vibrational spectra at the same theory level.
The nature of the obtained stationary points was checked through
analysis of the corresponding Hessian matrix. Calculations were
are defined by the C
5
-C
6
and C
6
8
-O bonds.
4
6
carried out using the Gaussian 03 suit of programs.
To identify the minimum energy conformations of 1-TE, a
systematic investigation on the potential energy surface of the
molecule was undertaken using the B3LYP/6-311++G(d,p)
method. This search was carried out for each of the two
tautomers. Table 1 displays the calculated relative energies
A set of internal coordinates was defined, and the Cartesian
force constants were transformed to the internal coordinate
space, allowing ordinary normal-coordinate analysis as described
4
7
by Schachtschneider and Mortimer. These calculations were
made using the program BALGA and the optimized geometries
and harmonic force constants resulting from the DFT(B3LYP)/
(
including zero point energy corrections) of the different
conformers found. The most stable conformer of each tautomer
is represented in Figure 1. The full set of conformers is shown
in Figures S1 and S2 (Supporting Information); their location
on bidimensional potential energy surface (PES) maps obtained
by performing relaxed scans on the PES of each tautomer are
shown in Figures S3 and S4 (Supporting Information). In
6-311++G(d,p) calculations. The calculated harmonic frequen-
cies (scaled with the factor 0.978, which has been shown to be
an appropriate choice when the theoretical level and basis set
1
9-23
here applied are used)
were used to assist the analysis of
the experimental spectra and to account for the zero-point
vibrational energy (ZPVE) corrections.
building these maps, the C
5
-C
6
-O
8
-H14 and N
4
-C
5
-C
6
-O
8

1
3078 J. Phys. Chem. A, Vol. 114, No. 50, 2010
Ismael et al.
TABLE 2: DFT(B3LYP)/6-311++G(d,p) Calculated APT Charges (q, in Units of Electron) on Selected Atoms for the Various
Conformers of 1-TEa
APT charges
tautomer
conformer
q (N
-0.353
-0.347
-0.345
-0.380
-0.380
4
)
q (N
-0.257
-0.264
-0.264
-0.273
-0.273
1
)
q (H14
)
q (H
+0.294
+0.287
+0.280
+0.269
+0.267
9
)
q (O
-0.632
-0.615
-0.616
-0.622
-0.617
8
)
1
H
H
I
II
III
IV
V
+0.267
+0.256
+0.257
+0.313
+0.311
(
(
-0.361 ( 0.018)
-0.300
(-0.266 ( 0.007)
-0.231
(+0.281 ( 0.029)
+0.293
(+0.279 ( 0.012)
+0.280
(-0.620 ( 0.007)
-0.625
2
I
II
-0.296
-0.228
+0.278
+0.283
-0.627
III
IV
V
VI
VII
-0.281
-0.257
+0.278
+0.282
-0.613
-0.279
-0.256
+0.273
+0.281
-0.608
-0.264
-0.238
+0.256
+0.279
-0.640
-0.262
-0.214
+0.245
+0.280
-0.632
-0.253
-0.219
+0.244
+0.275
-0.630
-0.276 ( 0.018)
(-0.235 ( 0.017)
(+0.267 ( 0.019)
(+0.280 ( 0.003)
(-0.625 ( 0.011)
a
Conformers are depicted in Figures S1 (1H forms) and S2 (2H forms). Numbers in parentheses correspond to mean ( standard deviation
values.
dihedral angles were varied in increments of 30 degrees and all
other geometric parameters were let free during the geometry
optimization. The optimized geometries for the various con-
formers are given in Tables S1 and S2 (Supporting Information).
On the whole, 12 conformers were found, 5 for tautomer 1H
and 7 for tautomer 2H.
As expected, the calculations yield the most stable conformer
of tautomer 2H as being the global minimum. However, the
energy of the most stable conformer of tautomer 1H was
predicted to be only 0.75 kJ mol- higher than that of the latter
species. Both tautomers have other low energy conformers:
forms 2H-II, III, IV, and V and 1H-II and III have calculated
relative energies to the most stable conformer of the corre-
1
-
1
sponding tautomeric form smaller than 5 kJ mol . The
populations of the different conformers in gas phase at room
temperature (25 °C), obtained according to the Boltzman
statistics and taking into account the DFT calculated relative
energies, are given in Table 1. Conformers 1H-I and 2H-I, II,
III, and IV are predicted to have populations above or about
Figure 2. Relative energy (including zero point correction) of the
conformers of tautomer 2H of 1-TE as a function of the calculated
APT charge on the hydroxyl hydrogen atom (H14). The line connecting
the points is for guiding the eyes.
1
0%, while conformers 1H-II and III and 2H-V should have
populations of about 5-4%. The remaining four conformers
were predicted to have negligible populations at room temper-
ature (RT). On the whole, tautomer 2H was predicted to account
for about 72% of the total equilibrium population versus ∼28%
of tautomer 1H. Table 1 also gives the populations expected
for the gas phase equilibrium at 50 °C (the temperature of the
vapor of the compound used to prepare the matrices in the
present study), which are equal, in practical terms, to those
characteristic of the RT gas phase. In particular, one can
conclude that only conformers 1H-I, II, and III and 2H-I, II,
III, IV, and V might be of experimental relevance for the present
study.
the 2H conformers is also a good indicator of the relevance of
the intramolecular OH · · ·N hydrogen bond to their relative
stability. In the case of tautomer 1H, the three most stable
conformers (1H-I, II, and III, with relative energies below 4 kJ
-
1
mol ) possess an intramolecular hydrogen bond of type
N-H· · ·O. Conformers 1H-IV and V bear an H-bond of type
OH· · ·N, but they have also an important repulsive interaction
between the ring hydrogen atom and the methyl and/or methine
hydrogen atoms and, as a result, they have relative energies
-
1
above 9 kJ mol .
Of great relevance for the matrix isolation study undertaken
in this investigation are the energy barriers separating the
different conformers. It is well-known that during deposition
of a cryogenic matrix conformational cooling may take place
if the barriers to conformational isomerization are low enough
The relative stability of the conformers can be correlated with
the dominant type of intramolecular interaction in each form.
In the case of tautomer 2H, the five conformers bearing an
intramolecular hydrogen bond of type OH · · ·N correspond to
-
1 48-50
(below a few kJ mol ).
We have shown that the extent of
-1
the most stable forms (with relative energies below 5 kJ mol ),
whereas conformers 2H-VI and VII, which do not have any
intramolecular H-bond, have relative energies of about 9 kJ
the observed conformational cooling depends on the temperature
of the cold substrate during matrix deposition, the higher the
4
8
temperature the more extended the conformational cooling.
-1
mol . In the two most stable conformers (2H-I and II), the
OH· · ·N bond involves the more negatively charged N atom
as electron donor (see Table 2), while in 2H-III, IV, and V, the
H-bond electron donor atom is N . The correlation found
between the APT charge on H14 (Figure 2) and the energy of
The relevant barriers for conformational conversion in 1-TE
were then evaluated at the DFT(B3LYP)/6-311++G(d,p) level
of theory. The results obtained for the 1H tautomer clearly
indicate that conformers 1H-II and III are separated from the
4
1
-1
most stable conformer 1H-I by energy barriers below 1 kJ mol

Photochemistry in 1-(Tetrazol-5-yl)ethanol
0.68 and 0.87 kJ mol-1 for 1H-II and III, respectively) and
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13079
(
should then convert to this latter form during matrix deposition.
The higher energy forms also have very low predicted energy
barriers to lower energy forms and, ultimately, shall also be
converted into the most stable form. In practical terms, these
results led one to expect an observation of a single conformer
of tautomer 1H in the matrix-isolation experiments: conformer
1
H-I. If the sublimated vapor of the compound used to prepare
the matrices correspond to an equilibrium state (both regarding
tautomerization and conformational interconversion) conformer
1
H-I should correspond to a population of about 28% of the
total population. For tautomer 2H, the calculations predict that
conformer 2H-II should convert to conformer 2H-I during
-
1
deposition (calculated energy barrier 1.89 kJ mol ) and
conformers 2H-IV and V to conformer 2H-III. The barrier
-1
between 2H-III and 2H-I is also low, 3.95 kJ mol , but still
large enough to preclude extensive conversion of the higher
energy form into the lower energy conformer during deposition
at the lowest temperatures attainable in our matrix-isolation set
up (10 K). The two highest energy conformers (2H-VI and VII)
also have barriers of isomerization to lower energy conformers
on the order of 4-5 kJ mol- and may in principle also be
trapped in the matrices deposited at 10 K. However, their
populations in the gas phase prior to deposition are certainly
below 1% and their unequivocal experimental detection looks
improbable. In summary, the theoretical results point to the
possibility of experimental observation of two main conformers
of tautomer 2H in the argon matrices deposited at 10 K,
conformers 2H-I and III and also to the eventual presence of
conformers 2H-VI and VII in trace amounts. If the sublimated
vapor of the compound used to prepare the matrices correspond
to an equilibrium state both regarding tautomerization and
conformational interconversion, the expected populations of
conformers 2H-I and III should be about 45 and 25%,
respectively.
1
Figure 3. Spectrum of 1-TE deposited in an argon matrix with the
substrate of the cryostat kept at 10 K during deposition and the
sublimation temperature equals to 50 °C (bottom), and calculated spectra
(
wavenumbers scaled by 0.978) for the most stable conformers of the
two tautomeric forms of the compound: 1H-I and 2H-I, together with
the spectrum resulting from the juxtaposition of the calculated spectra
of the two forms. In the experimental spectrum, the negative band at
-1
2
about 667 cm is due to traces of atmospheric CO . Note the different
scale interval before the break in the calculated versus experimental
spectra. The calculated spectra are simulated by Lorentzian functions
centered at the calculated frequency and with fwhm (full width at half-
Infrared Spectra of the Matrix-Isolated Compound:
Selection of Conformational States. In both tautomeric forms
-
1
maximum) equal to 2 cm
.
1
(C symmetry), 1-TE has 36 fundamental vibrations, all active
in the infrared. The results of the theoretical vibrational
calculations undertaken in the present investigation are presented
in the Supporting Information: Tables S3 and S4 display the
sets of internal coordinates used in the normal coordinate
analysis performed on tautomers 1H and 2H, respectively, and
Tables S5-S13 the calculated infrared spectra and potential
energy distributions (PED) for the lower energy (∆E < 5 kJ
of 2H-III into 2H-I during deposition of the matrix with the
substrate kept at 10 K, but allows its easy conversion upon
annealing of the matrix at a higher temperature. Very interest-
ingly, the ratio of the populations of 2H-I and 2H-III obtained
from the intensities of the observed bands ascribable to each of
these conformers in the as-deposited matrix spectrum was found
to be ∼2.5, which is significantly larger than that predicted based
on the calculations for the gas phase conformational equilibrium
at the sublimation temperature: 1.8. This result demonstrates
that during deposition of the matrix (10 K), the 2H-III form
partially converts to the most stable 2H-I conformer. On the
other hand, when the matrix was deposited with the substrate
kept at 30 K, extensive conformational cooling took place and
conformer 2H-III could not be observed, being completely
converted to 2H-I (the spectrum of the matrix deposited with
substrate at 30 K is identical to that resulting from the annealing
at 30 K of the matrix deposited with the substrate at 10 K).
There is a second group of bands present in the spectrum of
the as-deposited matrix at 10 K that also disappeared upon
annealing and that cannot be assigned to conformer 2H-III (e.g.,
-1
mol ) conformers.
The infrared spectrum of 1-TE isolated in an argon matrix
with substrate kept at 10 K during the matrix deposition is
presented in Figure 3, together with the calculated spectra for
the most stable conformers of each 1-TE tautomer. The spectrum
resulting from the juxtaposition of the two calculated spectra is
also shown in this figure. As it can be noticed, this latter
spectrum nicely fits the experimentally obtained one, indicating
that conformers 1H-I and 2H-I are the main contributors to the
observed spectrum, as suggested by calculations. Upon annealing
of the matrix to 30 K, some bands were found to vanish. These
bands are observed in several spectral regions, such as, for
example, that presented in Figure 4. One group of these bands
fits nicely the predicted IR spectrum of conformer 2H-III (e.g.,
bands around 570 and 715 cm- in Figure 4). As mentioned
above, this conformer could also be expected to be present in
the matrix deposited at 10 K with a significant population: the
relatively low energy barrier for the 2H-III f 2H-I conversion,
-
1
the band at ca. 610 cm ; see Figure 4). These bands appear
systematically at close positions to other bands that are given
rise by 1H-I and are most probably due to this species trapped
in a different, less stable matrix site. As for the bands due to
2H-III, the bands due to the less stable matrix site of 1H-I were
not observed in the spectrum of the matrix deposited at 30 K.
1
-1
3
.95 kJ mol , is still large enough to prevent full conversion

1
3080 J. Phys. Chem. A, Vol. 114, No. 50, 2010
Ismael et al.
Figure 4. (Bottom) Regions 730-700 and 630-570 cm- of the as-deposited IR spectrum of 1-TE with substrate at 10 K (thick line) and of the
spectrum obtained after annealing of the matrix at 30 K (thin line); (Top) calculated spectra (wavenumbers scaled by 0.978) for forms 1H-I, 2H-I,
and 2H-III. The bands due to 2H-III as well as to the less stable matrix site for 1H-I vanish in the spectrum of the annealed matrix (see text for
discussion).
1
A very interesting observation is the fact that the observed
relative populations of the two tautomers of 1-TE in the matrices
is considerably different from that predicted by the calculations
for an equilibrium situation in the gas phase at the temperature
of sublimation of the compound. Indeed, the theoretically
predicted equilibrium 2H-/1H- population ratio at 50 °C is 2.5,
whereas the observed one in the matrices is only ∼1.3 (as
obtained from the absorbances of the bands due to 2H- and
the experimental observations we now present for 1-TE, that
the relative gas phase populations of tautomers produced from
a crystal containing a single tautomer via thermal vaporization
cannot be accurately predicted without considering intermo-
lecular hydrogen bonding interactions present in the condensed
phase. Further, as a general case, these populations do not
correspond to the expected gas phase equilibrium populations
involving all tautomeric forms.
1
H- species in the experimental spectra, normalized by the
In summary, analysis of the matrix spectra of 1-TE obtained
using different temperatures of the cold substrate during
deposition of the matrices together with the performed annealing
experiments allowed the identification of the vibrational signa-
tures of forms 1H-I, 2H-I, and 2H-III, in agreement with the
theoretical predictions. The band assignments are given in Table
3. Form 2H-III was partially converted to 2H-I upon deposition
of the matrix at 10 K and it was totally converted into this latter
form when deposition was made with the substrate at 30 K.
Annealing of the 10 K deposited matrix to 30 K also promoted
complete isomerization of 2H-III into 2H-I. Hence, matrix-
isolation of 1-TE allowed an efficient selection of the most stable
conformer of each of its two tautomeric forms. On the other
hand, the population of the 1H-tautomer trapped in the matrices
was found to be considerably larger than that expected for the
gas phase equilibrium at the sublimation temperature. The 1H-
tautomer is the one existing in the crystal of the compound and
the observed increased population of 1H-form indicates that the
tautomeric equilibrium could not be reached during matrix
deposition, though partial conversion of tautomer 1H- into 2H-
took place.
corresponding theoretically predicted infrared intensities; spectral
regions used for these measurements: 560-620, 710-730,
1
the two tautomers were not in equilibrium in the sublimated
vapor of the compound. As generally observed for tetrazoles,3
in the crystalline sample 1-TE exists only in the more polar
-1
500-1580, and 3570-3700 cm ). This result indicates that
0-32
1
H-tautomeric form. Upon sublimation, this tautomer then
converts to the 2H- form, which is the most stable form in the
gas phase. The tautomerization barrier is, however, large (in
-1 51
tetrazole it is ca. 230 kJ mol ), so that under the high-vacuum
conditions of the experiment, the equilibrium could not be
reached along the few centimeters path separating the sample
sublimation chamber from the cold substrate.
The question of tautomerization during sublimation of a
compound existing in a single tautomeric form in the crystalline
state has been recently addressed by Yang and Rodgers52 for
the case of cytosine (also of some relevance is the manuscript
5
3
by Colominas et al., on the tautomerism of cytosine and
guanine). In the crystalline state, cytosine exists in a single
tautomeric state (the amino-oxo form),
54,55
but both in gas phase
and in cryogenic matrices it exists in several tautomeric species,
with the amino-hydroxy tautomer being the most stable
species.5 Like for 1-TE, barriers of energy for tautomerization
The possibility of efficient sellection of only one conformer
of each tautomer of 1-TE, having different types of dominant
intramolecular H-bond interactions (NH · · ·O in 1H-I and
OH· · ·N in 2H-I; see Figure 1) was crucial for the photochemi-
cal studies described in the next section and stresses the power
of matrix isolation to address photochemical problems where
specific isomers of the studied compound are under scrutiny.
UV-Irradiation Experiments (λ > 200 nm): Tautomer
Selective Photochemistry in 1-TE. For the photochemical
experiments, deposition of the matrix was undertaken with the
substrate at 30 K in order to select only forms 1H-I and 2H-I
6,57
-1 52
in cytosine are high (over 140 kJ mol ). Based on an extensive
series of calculations undertaken both at the MP2 and
DFT(B3LYP) levels of theory, Yang and Rogers proposed that
the tautomerization involves formation of dimers with adequate
intermolecular hydrogen bonding interactions during sublimation
of the compound, which results in a significant lowering of the
barriers to tautomerization and facilitates tautomerization during
5
2
the sublimation process. They concluded, in consonance with

Photochemistry in 1-(Tetrazol-5-yl)ethanol
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13081
-1
TABLE 3: Experimental and Calculated [B3LYP/6-311++G(d,p); Scaled by 0.978] Vibrational Frequencies (ν; cm ) and
-1
a
Calculated IR Intensities (I, km mol ) for the Observed Conformers of 1-TE Isolated in Argon Matrix
calculated 1H-I
approximate
description
2H-I
approximate
description
2H-III
approximate
description
b
observed ν
ν
I
ν
I
ν
I
3
3
3
3
3
3
3
664/3658/3653
621
611/3588/3586
485/3480/3477
469/3467/3462
463
νO-H
3763
47.0
νO-H
3729.4
38.3
νO-H
νN-H
3713.9
3551.6
49.6
νN-H
3567
114.9
121.6
νN-H
3554.2
3058.8
3049.9
125.6
10.3
19.4
010/3004/2999/2995
νCH
νCH
νCH
3
3
3
as′′
as′
s
3054
3035
7.6
νCH
νCH
3
as′
as′′
3055.2
3051.5
13.6
18.7
νCH
νCH
3
as′′
as′
3
3
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
9
9
8
7
7
7
7
7
7
6
6
6
6
5
5
987
20.3
950
νCH
3
s
2980.0
2901.3
11.0
34.6
νCH
3
s
2978.9
2902.6
12.3
31.8
941
917
2966
2937
18.3
19.5
νC-H
908/2902/2896/2890/2883
566/1563/1559
517/1513/1510/1504/1501
472
460
456/1447
450
νC-H
νC-H
νCdN
νCdN
1552
23.9
νCdN
δN-H
1505.3
1465.8
12.8
1.4
1500.3
1469.2
15.1
0.4
δCH
3
as′
δCH
3
3
as′
1461
1456
2.0
8.8
δCH
3
3
as′′
as′
1458.3
1439.1
8.0
2.1
δCH
3
as′′
1457.0
1436.9
1386.1
9.3
5.1
δCH
as′′
439/1436
434
403
395
391
δCH
δN-H
νC-N
1411
3.6
δCH
3
s
1395.3
23.1
δCH
3
s
21.8
387
δC-H′′
δCH s
3
1386
1378
27.2
9.9
376
366
360
334
330
δC-H′′
δC-H′
1369.5
1328.5
24.5
20.1
δC-H′′
δC-H′
1357.6
1343.8
34.1
9.6
δC-H′
νNdN
1325
1313
5.5
10.2
307/1305
290/1288/1284
268/1264
260/1258
252
νC-N
δC-O-H
1289.8
1260.7
26.5
60.4
νC-N
1283.0
22.8
δC-O-H
νNdN
1256.9
1234.8
54.3
2.6
νNdN
1235.3
7.2
243
δN-H
δC-O-H
1233.1
1183.8
15.9
81.2
219/1184
135/1133
126/1122
117
113
094/1092
079
νN
1
-N
2
3
1159.1
1120.5
16.3
46.3
νN
νC-O
νN -N
1
-N
2
1163.2
1116.7
1094.1
28.1
29.1
12.2
νC-O
γCH
3
′′
1106.3
81.8
νN
2
-N
1094.8
14.2
2
3
νC-C
7
1083.4
1074.9
15.5
7.9
074
070
δ(ring 2)
δ(ring 1)
γCH
3
′′
1069.3
1049.2
23.8
22.8
γCH
3
′′
1071.1
1048.1
11.0
59.6
054/1050
049
032/1030
026
δ(ring 1)
δ(ring 1)
1053.7
1020.8
1008.9
976.9
23.8
28.3
2.5
δ(ring 2)
1023.4
9.9
νN
γCH
νN -N
νC-O
1
-N
2
020
010
δ(ring 2)
1020.2
1002.1
899.5
38.9
8.6
3
′
008/1006/1000
70/966
06 (?)
γCH
3
′
997.0
895.2
782.2
41.4
10.7
1.7
γCH
3
′
3
4
7.4
νC-C
7
νC-C
7
9.8
92/891/889
89
88
52
24
18/716
15/714/713
69
877.4
19.8
τ(ring 2)
τ(ring 1)
τ(ring 2)
τ(ring 1)
775.6
1.6
τ(ring 2)
τ(ring 1)
743.9
722.3
2.9
20.5
714.3
648.7
581.2
41.8
14.8
51.9
714.6
656.2
572.1
36.8
13.4
54.7
νC-C
5
654.3
605.8
18.7
51.8
61
νC-C
5
57/673
17/610/607/604
97/593/590/584
86/580/576//572/569
νC-C
5
γN-H
γN-H
γN-H
a
ν, stretching, δ, bending, γ, rocking, τ, torsion, s, symmetric, as, antisymmetric. See Figure 1 for atom numbering and Supporting
b
Information for definition of internal coordinates and detailed normal coordinate analysis. Values in italic correspond to the less stable site for
H-I (see text).
1
as starting reactants. The matrix was then irradiated with UV
light provided by a Hg(Xe) lamp through a quartz window (λ
Upon irradiation, the bands due to tautomer 1H did not change
their intensity, even after prolonged irradiation (8 h), while those
due to tautomer 2H decreased (Figure 5). These results clearly
>
200 nm).

1
3082 J. Phys. Chem. A, Vol. 114, No. 50, 2010
Ismael et al.
Figure 5. (Top) Regions 730-700 and 630-570 cm- of the as-deposited IR spectrum of 1-TE with substrate at 30 K (thick line) and of the
1
spectrum obtained after irradiation of the matrix during 8 h with UV light (λ > 200 nm; thin line). The band marked with the asterisk belongs to
1
-TE dimer. (Bottom) Calculated spectra (wavenumbers scaled by 0.978) for forms 1H-I and 2H-I. While the bands due to the 1H-tautomer do not
change intensity upon irradiation, those due to the 2H-tautomer considerably reduce their intensity, evidencing that the observed photofragmentation
reactions are tautomer selective (see text for discussion).
indicate occurrence of an unprecedented tautomer selective
photochemistry for the UV-irradiated (λ > 200 nm) matrix-
isolated 1-TE, with the 2H-tautomeric form undergoing uni-
molecular decomposition easily and the 1H-tautomer being
photostable under the same experimental conditions.
Simultaneously with the consumption of the 2H-tautomer of
1
-TE, new bands appeared in the spectrum, which are due to
the photoproduced species. The spectroscopic results are
presented in Figures 6 and S5, which present the higher and
lower frequency spectral regions, respectively. In these figures,
of the difference IR spectra were obtained by subtracting the
spectrum of the as-deposited matrix (substrate temperature
during deposition: 30 K) to the spectrum of the matrix irradiated
during 8 h (λ > 200 nm). The calculated spectra for the 1H-
and 2H-tautomers and those of the photoproducts are also shown
for comparison. A schematic summary of the observed photo-
processes is given in Scheme 1.
In the difference spectrum shown in Figures 6 and S5, the
bands due to tautomer 1H are not observed neither as positive
bands nor as negative, because this species is nonreactive.
Negative bands in this spectrum correspond to the tautomer 2H,
whereas the positive ones, due to the photoproducts, could be
fully assigned to azide (N
abbreviated as 2HPN), which are the primary products of the
reaction, and HCN and acetaldehyde, which were produced from
HPN in a secondary photoreaction. The higher frequency
3
H) and 2-hydroxypropanenitrile
(
2
spectral region (see Figure 6) is particularly clear for product
identification.
In this spectral range, (i) azide gives rise to two bands,
appearing as a doublet at 3415/3412 cm- (N-H stretching:
1
Figure 6. (Bottom) Calculated IR spectra of the most stable conformers
of 1-TE tautomers (1H-I and 2H-I) in the high frequency region;
(middle) difference IR spectrum, irradiated matrix during 8 h with λ >
νNH) and as a very intense structured band with maximum at
2
142 cm- (NdNdN asymmetric stretching: νNdNdN as).
1
2
00 nm minus as-deposited matrix (substrate temperature during
These bands were observed in the spectrum of azide isolated
-
1
58
deposition: 30 K); (top) calculated spectra of the observed photoprod-
ucts. Note the different scale interval before the break in the calculated
vs experimental spectra. The calculated spectra are simulated by
Lorentzian functions centered at the calculated frequency (scaled by
in argon matrix at 3336 and 2140 cm , respectively. The
considerable deviation in the observed azide νNH band com-
pared with the literature value for the sole matrix-isolated
compound indicates that, once produced from fragmentation of
-1
0
.978) and with fwhm (full width at half-maximum) equal to 2 cm
-
1
1
-TE, the azide molecule interacts in the matrix cage where it
(1-TE) or 4 cm (photoproducts).

Photochemistry in 1-(Tetrazol-5-yl)ethanol
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13083
SCHEME 1: Simplified Scheme Showing the Observed
the other species produced in the same matrix cage (acetalde-
hyde, azide). The corresponding frequencies for HCN monomer
Photoprocesses Resulting from UV Irradition (λ > 200
a
-
1 62,63
nm) of Matrix-Isolated Monomeric 1-TE
isolated in argon matrix are 3306 and 2098 cm .
On the
other hand, the most intense band of HCN (bending mode)
appears in the low frequency spectral region of the spectrum of
-
1
the 1-TE photolyzed matrix at 724 cm (see Figure S5), that
is, close to the frequency of the HCN monomer in argon matrix,
-
1 62,63
7
21 cm .
The complete assignment of the bands observed for the
different photoproducts are given in Table 4, together with
58-63
availablepreviouslyreportedvaluesfortheisolatedcompounds
and calculated data.
The photochemically induced opening of the tetrazole ring
has been found to be in general an easy and considerably fast
process. Under identical experimental conditions to those used
in the present study, all tetrazole derivatives we studied
22-25
previously were found to react extensively in a few minutes.
In this regard, the observed ring-opening reaction in 1-TE is
slower, with only about 50% of the reactive tautomer initially
present being consumed after 8 h of irradiation. The reasons
for the relatively inefficiency of the photochemical process as
well as its tautomer selectivity must then be related with the
characteristics of the substituent present in the tetrazole ring.
a
Tautomer 1H was found to be photostable under these conditions.
The calculated HOMO/LUMO gap was found to be consider-
ably different in the two tautomers, amounting to 7.12 eV (174
nm) in the case of tautomer 1H, and 6.51 eV (191 nm) for
tautomer 2H. This difference results mainly from the fact that
in tautomer 1H the tetrazole hydrogen atom is involved in
H-bonding, which will decrease electron-density on the tetrazole
ring, whereas in tautomer 2H no such effect is observed. The
HOMO/LUMO gap values indicate that the performed excitation
Tautomer 2H undergoes unimolecular photodecomposition to azide and
-hydroxypropanenitrile, with the latter compound undergoing subse-
2
quent fragmentation into HCN and acetaldehyde. The different types
of intramolecular H-bond in the two tautomers are proposed as relevant
to their different photochemical behavior. A higher energy conformer,
such as 2H-III, where no intramolecular H-bond exists connecting the
two photoproduced fragments, is proposed as intermediate in the
process, with the internal rotation converting 2H-I into this form taking
place in an excited state.
(
λ ) 200 nm) was made in the low energy wing of the
absorbance band, considerably far from the absorbance maxi-
mum corresponding to tautomer 1H. This is certainly an
important factor for the observed photostability of this tautomer
under the conditions employed in this work, and it may also
justify, at least partially, the relative inefficiency of the process
for tautomer 2H. However, another factor seems also to be
relevant to explain these observations. In the case of tautomer
is formed with other photoproducts (2HPN, HCN, or acetalde-
hyde). Note that the presence of azide in the photolyzed matrix
is further reinforced by the observation of the band due to the
N-H deformation mode in the low frequency spectral range
-1
58
(
see Figure S5) at about 1130 cm . According to the literature,
-1
this band occurs at 1150 cm in the argon isolated azide.
-1
-1
(ii) 2HPN absorbs at 3625/3622 cm (νOH) and 2254 cm
1H, all low-energy conformers (and in particular the most stable
(
νC≡N), which are very typical frequency values for O-H
59,60
one; 1H-I) have an intramolecular H-bond connecting the
tetrazole hydrogen to the oxygen atom of substituent. The ring-
opening reaction then requires simultaneous cleavage of this
bond (Scheme 1), which appears to be a blocking factor for the
reaction. On the other hand, in the case of tautomer 2H, there
are low energy conformers that can be easily accessible from
the lowest energy conformer in which the intramolecular H-bond
is not required to be cleaved during the ring-opening reaction,
in particular conformer 2H-III (but also 2H-IV and 2H-V). In
these conformers, the ring-opening reaction is then much easier,
since the two molecular fragments resulting from the reaction
are not linked by any H-bond (see Scheme 1). Note that this
effect acquires an increased importance when the reaction takes
place in a matrix, because in this case the photoproduced
fragments tend to stay spatially close (diffusion is restricted
under these conditions) and the probability of recombination
increases. In addition, because the initial conformation of the
reactant is 2H-I, the photochemical reaction requires internal
rotation to take place previously in order to form the reactive
conformers (e.g., 2H-III, whose energy is very similar to that
of 2H-I and is separated from this latter form by a small energy
barrier in the ground electronic state and, with all probability,
alcohol and C≡N stretching modes.
Note that 2HPN can
occur in three different conformers differing in the orientation
of the O-H bond. The most stable conformer has the O-H
bond nearly anti to the methyl group and is more stable than
the conformers with the O-H bond anti to the H atom and to
-1
the C≡N group by 1.0 and 8.5 kJ mol , respectively. However,
because the barriers for the conversion of the two less stable
conformers into the most stable conformer are low (0.64 and
-1
4
.8 kJ mol ), once formed, these species shall decay promptly
to this latter. Note that most of the bands expected for 2HPN
in the low frequency region were also observed in the spectrum
of the 1-TE photolyzed matrix (see Figure S5).
(
iii) Acetaldehyde gives rise to the band observed around 1731
-1
cm (νCdO), which fits nicely the previously reported value
for the isolated acetaldehyde molecule in argon matrix, 1727
-1 61
cm . The broad shape of this band (see Figure 6) indicates
interaction of acetaldehyde in the matrix cage where it was
formed either with HCN or azide.
(
iV) Finally, in the high frequency range, HCN is in the origin
-1
-1
of the bands at 3406/3403 cm (νCH) and 2080 cm (νC≡N).
As expected, the observed frequencies for these two modes
indicate involvement of the HCN molecule in complexes with

1
3084 J. Phys. Chem. A, Vol. 114, No. 50, 2010
Ismael et al.
TABLE 4: Experimental and Calculated [B3LYP/
of 1-(1H-diaziren-3-yl)ethanol), which is an usual relevant
photoprocess for tetrazole derivatives.2
2-25
6
-311++G(d,p); Scaled by 0.978] Vibrational Frequencies
-1 -1
(
ν; cm ) and Calculated IR Intensities (I, km mol ) for the
Observed Photoproducts of 1-TE Monomer (2H-tautomer)
Conclusion
Isolated in Argon Matrix
In this work, a combined matrix isolation FTIR and theoretical
study of the molecular structure and photochemistry of 1-(tet-
razol-5-yl)ethanol [1-TE] was performed.
observed
calculated
literature data
ref
approximate descriptiona
ν
ν
I
ν
The DFT(B3LYP)/6-311++G(d,p) potential energy surface
of the two tautomers of the compound were investigated and
used to characterize the conformational mixture existing in
equilibrium in gas phase prior to deposition of the matrices as
well as the conformers trapped in these latter. In gas phase, at
room temperature, the compound exists as a mixture of 12
conformers (five of 1H tautomer and seven of 2H tautomer),
from which only three main forms survive after deposition of
the compound in an argon matrix at 10 K. This conformational
selection could be achieved since the barriers for conformational
isomerization allow extensive conformational cooling to take
place during deposition. Deposition of the matrix at 30 K led
to further simplification of the conformational mixture with only
one conformer of each tautomer of 1-TE surviving. These
conformers correspond to the most stable forms of each tautomer
2
-hydroxypropanenitrile (2HPN)
3625/3622 3738 45.5
νOH
νCH
νCH
νCH
νCH
νCN
3
′′ as.
′ as
s
2990
2990
2944
3062 10.7
3058 10.6
3
3
2983
7.5
2879/2866 2936 20.5
2254
n.obs.
1449
2292
1464
1452
1.6
2.9
6.7
9.4
δCH
δCH
δCH
3
′ as
′′ as
s
3
3
1381/1376 1385
δOH
δCH
γCH
νCO
1357
1352
1259
1113
1064
1018
919
1352 12.8
1335 23.1
1248 51.2
1111 41.9
γCH
3
′′
1061
8.2
νC-C
1019 66.1
910 19.9
γCH ′
3
(1H-I and 2H-I forms). A complete assignment of the IR spectra
νC-C
793
594
n.obs.
n. i.
786
582
551
386
4.6
1.4
3.7
9.5
γC-C≡N
δC-C≡N
δC-C-O
τC-OΗ
δCCC
of 1H-I, 2H-I, and 2H-III conformers was performed taking also
into account the results of the normal coordinate analysis
performed for this species.
n. i.
315 138.4
Upon irradiating with UV light (λ > 200 nm) a matrix
containing both 1H-I and 2H-I forms, an unprecedented tautomer
selective photochemistry was observed, with the 2H- tautomeric
form undergoing unimolecular decomposition to azide +
hydroxypropanenitrile, and the 1H-tautomer being photostable.
The tautomer selective nature of the observed photoprocess was
interpreted based on the characteristics of the substituent and
different types of intramolecular H-bonds characterizing the
most stable conformers of each tautomer.
n. i.
n. i.
n. i.
264
216
182
9.6
1.0
9.0
τC-CH
γCCC
3
Azide
3415/3412 3412 47.2 3336 58
νNH
νNdNdN as
δNH
2142
1263
1132
610
2218 406.1 2140 58
1266 0.2 1274 58
1148 223.0 1150 58
νNdNdN s
γNdNdN
δNdNdN
578
0.6
607 58
522 58
n.obs.
524 16.0
Acknowledgment. The authors are grateful to Funda c¸ a˜ o para
a Ci eˆ ncia e Tecnologia [Projects PTDC/QUI/67674/2006 and
PTDC/QUI/71203/2006, also supported by COMPETE and
FEDER], ANCPCyT (Project PICT(2006)/00068), bilateral
cooperation grant (FCT-MinCyT PO/09/18), and CYTED
Acetaldehyde
νCH
νCH
νCH
3
3
3
′ as.
′′ as
s
2990
n.obs.
2930
2768
1731
1436
1428
1387
1355
n.obs.
1078
906
3053 17.8 3023 61
3029
2966
2814 130.7 2715
1763 197.8 1727 61
4.2 2962 61
1.9 2922 61
b
νC-H
61
νCdO
δCH
δCH
(Network 108RT0362) for financial support. A.G.-Z. is member
b
3
3
′′ as
′ as
1442
9.8 1436
61
of the Research Career CONICET, Argentina.
1436 14.5 1427 61
1393 15.4 1390 61
1343 17.3 1349 61
δC-H
δCH
γC-H
γCH
νC-C
γCH ′′
Supporting Information Available: Figures S1 and S2, with
the structures of the conformers of 1H- and 2H-tautomeric forms
of 1-TE; Figures S3 and S4, with potential energy surface maps
of 1H- and 2H-tautomers of 1-TE showing the location of their
3
s
1107
0.3 1111 61
3
′
1071 38.2 1098 61
908 13.9
736 2.4
501 12.5
165
871 61
772 61
506 61
3
n.obs.
n. i.
n. i.
-1
possible conformers; Figure S5, showing the 1600-550 cm
δCC)O
τC-CH
7.2 144b
spectral range of the infrared difference spectrum (irradiated
3
61
1
-TE matrix with λ > 200 nm for 8 h minus as-deposited 1-TE
HCN
3406/3403 3380 67.3 3306 62,63
matrix) and the calculated spectra for tautomers 1H-I and 2H-I
and observed photoproducts; Tables S1 and S2, with optimized
geometries of the conformers for tautomers 1H- and 2H,
respectively; Tables S3 and S4, with definition of the internal
coordinates used in the normal coordinate analysis performed
on the different conformers of 1H- and 2H- tautomers of 1-TE;
Tables S5-13, with calculated IR spectra and results of the
νCH
νC≡N
2080
724
2148
747 84.7
1.9 2098 63
721 62,63
δHC≡N
a
ν, stretching; δ, bending; γ, rocking; τ, torsion; n.obs., not
b
observed; n. i., not investigated. Gas phase data.
normal coordinate analysis for the for the lowest energy (∆E <
also in the bright excited state). Such fact may also contribute
to the observed relative inefficiency of the photochemical
process.
-1
5
kJ mol ) conformers. This material is available free of charge
via the Internet at http://pubs.acs.org.
A final note must be made in relation with the photochemical
results. This is related with the nonobservation for 1-TE of
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