Vibrational spectra, conformations, and intramolecular interactions of R(CH2)2-O-PCl2 molecules (R = Et, OMe)

Article abstract of DOI:10.1016/S0022-2860(98)00496-7

IR and Raman spectroscopy, normal coordinate analysis, and molecular mechanics were used to study the conformations of R(CH₂)₂-O-PCl₂ molecules (R = Et, OMe). It is shown that in the case of R = Et, that the molecules prefer to exist in more or less extended conformations, R and PCl₂ groups being spatially far apart. In contrast, the convoluted conformations are preferred in the case of R = OMe. In these conformations the intramolecular distances between nonbonded P1 and O5 atoms are comparable with the sums of the corresponding van der Waal's radii. The substantial energetical preference of the convoluted conformations in the liquid state is assigned to certain attractive intramolecular 1-5 interactions between the phosphorus and the oxygen atoms. The energy of the interaction may amount to ~2-4.5 kcal mo^-1.

Full text of DOI:10.1016/S0022-2860(98)00496-7

MOLSTR 10389
Journal of Molecular Structure 475 (1999) 13–25
Vibrational spectra, conformations, and intramolecular interactions of
R(CH ) -O-PCl molecules (R = Et, OMe).
2
2
2
a
,
b
a
a
a
S.A. Katsyuba *, R. Schmutzler , L.V. Avvakumova , A.E. Vandyukov , V.A. Alfonsov ,
b
C. Melnicky
a
A.E. Arbuzov, Institute of Organic and Physical Chemistry, Kazan Scientific Centre of Russian Academy of Sciences, Arbuzov str. 8,
4
20088 Kazan Russia
b
Institut f u¨ r Anorganische und Analytische Chemie der Technischen Universit a¨ t, Postfach 3329, D-38023 Braunschweig, Germany
Received 2 March 1998; accepted 6 April 1998
Abstract
IR and Raman spectroscopy, normal coordinate analysis, and molecular mechanics were used to study the conformations of
R(CH -O-PCl molecules (R = Et, OMe). It is shown that in the case of R = Et, that the molecules prefer to exist in more or
less extended conformations, R and PCl groups being spatially far apart. In contrast, the convoluted conformations are
2
)
2
2
2
preferred in the case of R = OMe. In these conformations the intramolecular distances between nonbonded P1 and O5
atoms are comparable with the sums of the corresponding van der Waal’s radii. The substantial energetical preference of
the convoluted conformations in the liquid state is assigned to certain attractive intramolecular 1–5 interactions between the
−1
phosphorus and the oxygen atoms. The energy of the interaction may amount to ϳ2–4.5 kcal mol . ᭧ 1999 Elsevier Science
B.V. All rights reserved.
Keywords: Vibrational spectra; Conformations; nonbonded heteroatoms
1
. Introduction
Me, t-Bu, Ph, NEt ; X = NMe, O) were described in
2
a recent review [2]. In these molecules the phosphorus
atom appeared as acceptor atom and the nitrogen atom
functioned as a donor, which lead, in certain cases, to
Nt heb fo on r dm. ation of the intramolecular coordinative P ←
The intramolecular interactions of nonbonded
heteroatoms attract considerable attention in the con-
text of present interest in hypervalency at phosphorus,
silicon, etc. [1]. They could be regarded as the first
step in chemical bond formation. The compounds
exhibiting these interactions may be used as models
of prereaction and transition states of nucleophilic
substitution reactions. Several examples of such a
kind of interaction between P(III) and N atoms of
Me NCH CH -X-PYZ molecules (Y, Z = Cl, Br,
Our recent studies [3] of the related molecule Cl P-
2
O-CH CH SCN (1) revealed the possibility of more
2
2
subtle intramolecular interactions between the sulfur
and the phosphorus atoms. The sulfur atom of the
thiocyanato group of the molecule 1 exhibited much
less pronounced donor ability than the dialkylamino
group. So, the corresponding interaction influenced
perceptibly only the relative energy of the conformers
with and without intramolecular contacts between the
2
2
2
*
Corresponding author. Tel: 007 8432751892, Fax: 007
432752253, E-mail: katsyuba@glass.ksu.ras.ru
8
0
022-2860/99/$ - see front matter ᭧ 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(98)00496-7

1
4
S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
heteroatoms. This energetical effect of the interaction
was evaluated to be ϳ2–4 kcal mol in the liquid
state [3].
spectrometer ‘IFS-113v’ (Bruker) and on the spectro-
meters ‘IR-75’ and ‘UR-20’ (Carl Zeiss) in the 400–
−1
−1
4000 cm range. The spectrum of CCl was sub-
4
We intend to undertake the conformational analysis
of the compounds of type R(CH ) -X-PYZ (X = O,
tracted from the spectra of the solutions. The spectra
of solids were recorded on the same instruments either
by condensing the samples on a KBr plate held at
boiling liquid nitrogen temperature or by cooling
liquid films between KBr plates. Repeated annealing
of the samples did not lead to their transformation
from glassy to cristal state.
Computations. For a detailed interpretation of the
experiments a normal coordinate analysis was carried
out, following the well-known GF-method [6]. The F-
matrices of all possible conformers of molecule 2
were additively composed of force constants trans-
2
n
NMe, S, etc.; R = OMe, NMe , etc.) to evaluate and to
2
compare the effects of the intramolecular interactions
between different heteroatoms in a different chemical
environment. It is hoped that variations in the number
of the methylene groups (n) could change the distance
between the heteroatoms, all the way to the formation
of a chemical bond. Thus it may be possible to follow
the elementary chemical process, and estimate the
relation between the energy of the interaction and
the distance between the interacting centers. The pre-
sent work is devoted to studies of R(CH ) -O-PCl
ferred from the molecules
1
[3] and
2
2
2
molecules, where R = Et (2), and OMe (3).
CH CH CH CH OCH (4) [7]. The combination of
3
2
2
2
3
force constants of the molecules 2 and CH OCH
3
2-
CH OCH (5) [7] was used for similar evaluations
2
3
2
. Experimental
in the case of 3. The force constants [7] were success-
fully used in the past for spectra simulation of mole-
cules 4 [8], 5 [9], and many other alkyl ethers [8].
The values of bond lengths and bond and dihedral
angles for the RCH CH O moieties of the molecules 2
All compounds were handled with exclusion of air
and moisture in sealed vessels or in a glove box.
Solvents were dried by standard procedures.
2
2
Compounds 2 and 3 were synthesized by the reac-
tion of PCl with C H OH [4], and MeOCH CH O-
and 3 were taken from Ref. [7], and for the Cl P-O-C
2
moiety — from those determined in the electron
diffraction study [10] of the CH OPCl molecule.
3
4
9
2
2
SiMe3 [5], respectively. They were investigated
immediately after repeated distillation in an atmo-
sphere of dry argon (b.p. 51ЊC at 10 Torr, and 55ЊC
at 9 Torr, respectively). The purity was controlled by
3
2
The ClPOC and POCC dihedral angles of the model
conformers were estimated by molecular mechanics
calculations. For that purpose the molecular
mechanics force field [11–13], successfully used for
the conformational analysis of ethyldichlorophosphite
[14] and many other organophosphorus molecules
(e.g. Ref. [15]), was applied.
Prior to its application, we optimized torsion term
parameters necessary to deal with ether fragments of
the molecules under study. Simulation of conforma-
tions of ethers, especially of 1,2-dimethoxyethane
(DME), has been the focus of theoretical studies in
the last few years (e.g. [16–20]). Several groups
[17,18,21] have shown that both large basis sets and
electron correlation corrections are necessary to repre-
sent the relative conformational energies accurately.
Considerable effort has been directed toward simula-
tion of high-level quantum chemical computations of
conformational energy within the framework of much
less time-consuming method of molecular mechanics
(e.g. [20]). In this paper we suggest only two new
1
31
IR, Raman, H and P NMR spectroscopy.
Raman spectra were recorded on a ‘Coderg’ spec-
˚
trometer with a He–Ne ‘LG-38’ laser (l = 6328 A,
power 50 mW), and on an ‘RTI-30’ spectrometer
+
˚
(
Dilor) with an Ar ‘ILA-130’ laser (l = 4880 A,
power 100 mW). The samples were sealed in glass
capillaries. Compounds 2 and 3 were studied at dif-
ferent temperatures. For this purpose the capillaries
were cooled in standard refrigerator unit with vapours
from liquid nitrogen. The amorphous samples
obtained in this way at boiling liquid nitrogen tem-
perature were annealed several times, but no changes
in the spectra were observed. Evidently the samples
were not able to transform from glassy to cristal state.
For depolarization ratio measurements liquids were
placed in a standard glass cell. IR spectra of the
pure compounds 2, 3 and of their solutions in CCl4
(
C Ϸ 0.1–0.01 M) were recorded on a FTIR

S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
15
−
1
parameters for general force field [11–13] to repro-
duce high-level ab initio and experimental results for
conformational energies of DME and some other
alkyl ethers.
0.10 kcal mol− [26]) and with the ab initio result
1
1.45 kcal mol
[17]. The calculated gauche,-
−1
gauche–anti,anti energy difference (2.61 kcal mol )
compare well with the ab initio value 2.94 kcal mol
−1
Within the framework of initial force field [11–13]
the torsional contributions to the potential energy
function do not take into account the oxygen gauche
effect [22]. So, we added a corresponding term
[17]. The energies predicted for anti,anti, gauche,-
gauche and gauche,anti conformers of methylpropyl
ether relative to most stable anti,gauche conformer
−1
are equal to 0.19, 1.12 and 1.49 kcal mol , respec-
tively. The first two values agree well with the ab
U Ј(OC-CO) = 0.5VЈ(1 + Cos2J), and adjusted VЈ
tor
−1
to reproduce the energy difference between ttt and
tgt conformations of DME, calculated at MP2/D95 +
initio evaluations (0.4 and 1.2 kcal mol [19]). To
the best of our knowledge the gauche,anti–anti,-
gauche energy difference has not been evaluated by
any method.
(2df,p) level [17]. Then the second additional term
U Љ(CC-OC) = 0.5VЉ(1 + CosJ), was introduced to
tor
adjust the ab initio [17] energy difference between ttt
So, simple modification of general force field [11–
13], proposed in this work, allows to reproduce accu-
rately the conformational properties of DME and
some other alkyl ethers. The computations were
made on the assumption that all the bonds are rigid.
−1
and ttg conformations of DME. VЈ = 1.25 kcal mol ,
−1
and VЉ = 1.35 kcal mol were found to be the optimal
values. A comparison of molecular mechanics and ab
initio conformer energies is given in Table 1.
To test the parameters, adjusted for DME, we have
also calculated the conformational energies of several
alkyl ethers. The computed value for the energy
difference between gauche and anti conformers of
3. Results
−1
ethylmethyl ether (1.2 kcal mol ) agrees well with
For the purpose of the conformational analysis of 2
and 3 we have studied their vibrational spectra (Table
2, Table 4). Our assignments in the frequency region
−1
the values of 1.1, 1.35 and 1.5 kcal mol determined
from the temperature dependence of the vibrational
−1
spectra of liquid [23], solution in CS [24] and gas
1200–3000 cm are based on the interpretation of the
2
[25], respectively. In the case of diethyl ether the
spectra of the molecules 1 [3], 4 [8] and 5 [9]. The
energy difference between anti,gauche conformer
and most stable anti,anti form is equal to 1.27 kcal -
mol , according to our calculations. The value quite
assignments of POC stretch and PCl scissor modes
2
are rather obvious because the corresponding bands
have typical frequencies, depolarization ratios and
high intensities [3,14,27]. All other bands are inter-
preted mainly on the basis of our normal-coordinate
calculations, which will be discussed later.
In the following subsections, the spectra and con-
formations of the individual molecules 2 and 3 are
discussed.
−1
agrees both with experimental evaluation (1.37 Ϯ
Table 1
Energies of DME conformers relative to the TTT conformer
a
a
Conformer
ab initio
Molecular mechanics
ttt
0.0
0.0
tgt
tggЈ
ttg
0.10
0.23
0.10
0.37
1.54
1.49
2.46
1.79
1.70
2.89
3.07
b
3.1. CH -CH -CH -CH -O-PCl (2)
3
2
2
2
2
1.43
1.51
1.64
1.86
2.41
3.08
3.13
tgg
The presence of conformers is apparent in the fre-
−1
ggg
gggЈ
gЈggЈ
gtgЈ
gtg
quency region below 1410 cm where more than
thirty IR bands and Raman lines are present in the
spectra of the liquid (Table 2). This number is much
larger, than the expected value (no more than 23) for
one conformer. ‘Extra’ bands exist in the spectra of
a
The notation of conformers and their energies are taken from
dilute CCl solutions of compound 2 and, hence, they
4
[17].
b
cannot be assigned to products of intermolecular asso-
ciation. The relative intensities of the components of
Experimental value for gas phase [21] is equal to 0.31 Ϯ
−1
0
.04 kcal mol .

1
6
S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25

S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
17

1
8
S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
several multiplets significantly change upon varying
the temperature (Fig. 1). Evidently, these changes are
caused by a shift of conformational equilibrium with
temperature.
According to molecular mechanics computations
molecule 2 is able to exist in 35 stable conformations
best fit to the observed frequencies of nPCl2 and
dPCl vibrations (Table 1) for all the model con-
2
formers simultaneously we slightly adjusted the
force constants of PCl bonds and ClPCl bond angle
manually. (The force constants of all the model con-
formers are available from the authors upon request.)
Normal coordinate results fit well with the above
suggestion that compound 2 is conformationally inho-
mogeneous (Table 2). True enough, the existence of
every particular conformer from Table 2 could not be
proved. Nevertheless, the components of the IR doub-
(Table 3), which can be distinguished by vibrational
spectroscopy. As described earlier, we calculated the
normal modes of all these conformers in order to
make the spectral interpretation more clear. The
calculated frequencies of the model conformers of
molecule 2 fit associated observed values quite satis-
factorily (Table 2), and additional refinement of the
initial force field was practically unnecessary. This is
scarcely surprising, because the chemical environ-
ments of the Cl POCH and CH CH CH moieties
−1
−1
let 714/722 cm (Raman 718/728 cm ) should be
assigned, based on our computations, to anti and
gauche conformations about the P–O bond, respec-
tively. Such an assignment can be confirmed by ab
initio calculations of the CH OPCl molecule [28],
2
2
2
2
3
3
2
of molecule 2 are almost identical to those of the
parent molecules 1 and 4. Hence, the transferability
of force constants is bound to be quite good, though
minor changes of OPCl2 force constants in com-
parison with the corresponding parameters of the
molecule 1 are possible [14,15]. So, to obtain the
according to which the gauche conformation about
the P–O bond must exhibit higher frequency in this
region, than anti conformation.
When liquid 2 is heated, the intensity of the band at
−1
714 cm decreases relative to the intensity of the
−1
band at 722 cm . Conversely, it increases upon cooling
4 9 2
Fig. 1. Temperature dependence of the infrared spectra of C H OPCl (2).

S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
19
−
1
of liquid 2 (Fig. 1). This is indicative of the energetical
preference of the aforementioned anti conformations
about the P–O bond in respect to gauche conformers.
From the temperature dependence of the intensity
ratio of these bands the enthalpy difference (DH)
between the two conformations was estimated.
Several measurements in the temperature range from
weak IR bands at 560, 587 and 612 cm to gauche
conformations about the C–O bond, and the strong
−1
band at 496 cm to anti conformations about the
C–O bond. The quantitative determination of the
enthalpy difference for the anti–gauche equilibrium
about the C–O bond was not possible, because the
origin of the band at 496 cm (anti conformations)
differed from the origin of the bands at 560, 587 and
−1
−1
3
5ЊC to − 5ЊC gave a value of 0.5 Ϯ 0.3 kcal mol
for this DH.
We can also assign with certainty at least one of the
−1
612 cm (gauche conformations). According to our
normal coordinate computations the former band is
Table 3
Energy and geometry characteristics of stable conformations of C
4
9 2
H OPCl (2), calculated by the molecular mechanics method
a
b
1
b
2
b
3
b
4
c
−1
No.
Name
f
(Њ)
f
(Њ)
f
(Њ)
f
(Њ)
U (kcal mol
)
1
2
3
4
5
6
7
8
9
AAAA
AAAG
AAGA
AAGG
AAGGЈ
AGAA
AGAG
AGAGЈ
AGGA
AGGGЈ
AGGG
AGGЈA
AGGЈGЈ
GAAA
GAAG
GAAGЈ
GAGA
GAGG
GAGGЈ
GAGЈA
GAGЈG
GAGЈGЈ
GGAA
GGAG
GGAGЈ
GGGA
GGGG
GGGGЈ
GGЈAA
GGЈAG
GGЈAGЈ
GGЈGG
GGЈGЈA
GGЈGЈG
GGЈGЈGЈ
180
179
188
188
189
150
147
148
148
185
148
150
153
66
67
67
66
67
65
65
61
61
41
71
69
44
49
72
66
65
68
65
66
63
180
178
193
196
200
66
69
68
70
107
72
180
178
60
56
69
177
178
179
60
64
59
−92
−92
180
178
182
60
180
63
180
61
−62
180
64
−64
184
−65
64
179
−65
180
63
−63
180
61
−62
180
60
−60
180
63
−63
182
60
−66
180
63
−64
62
180
64
0.96
0.89
0.58
0
0.22
2.45
2.24
2.64
1.62
1.72
1.85
4.04
3.98
1.98
1.14
1.28
0.96
0.80
0.89
0.87
0.50
0.40
3.31
2.15
2.40
2.23
2.24
1.60
2.42
2.51
2.31
1.71
1.94
1.72
1.89
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
79
74
182
184
186
184
187
185
181
163
169
78
56
67
−60
−69
−56
175
182
184
54
112
113
78
87
48
60
105
−71
−72
−69
−96
−67
−68
−67
180
178
181
61
−57
−64
−54
66
−61
a
Here A stands for anti, G for gauche.
, f , f , f — dihedral angles of rotation around the P–O, O–C3, C3–C4 and C4–C5 bonds. They are equal to zero in eclipsed position
1 2 3 4
bf
1 2 3 4
of the phosphorus lone pair and the OC bond (f ), PO and C3–C4 bonds (f ), OC and C4–C5 bonds (f ), C3–C4 and C5–C6 bonds (f ). For
the numbering scheme see Fig. 1.

2
0
S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
Table 4
3 2 2 2
Vibrational spectra of CH OCH CH OPCl (3)
Observed (liquid)
Calculated for the conformers
IR
Raman
AAGA (0.1) GAGA (0.0) AAAA (0.7) GAAA (0.8) AGGЈA(1.0) GGGЈA (0.8) Assigment for
GGGЈA
−1
−1
−1
−1
−1
−1
−1
−1
n (cm ) I
n (cm ) I r
n (cm
)
n (cm
)
n (cm
)
n (cm
)
n (cm
)
n (cm )
1
475 sh
1479
1466
1460
1451
1442
1388
1359
1479
1466
1460
1451
1442
1388
1359
1481
1466
1460
1451
1439
1400
1481
1466
1460
1452
1439
1405
1479
1466
1460
1452
1438
1402
1370
1479
1466
1460
1452
1438
1401
1370
dCH
wCH
2
1
1
1
1
1
1
1
1
1
468 w
460 sh
452 sh
447 m
401 w
363 m
333 vw
300 sh
284 m
2
3
d
s
CH
1453 w
d
asCH
3
dCH
2
wCH
2
1348
1278
1268
1348
1278
1268
1298
1298
1291
1291
tCH
2
1
254 sh
1
1
1
1
1
1
1
241 m
201 s
1254
1198
1154
1126
1254
1198
1154
1126
1251
1202
1155
1130
1069
1061
1251
1202
1155
1128
1071
1203
1157
1130
1084
1067
1203
1157
1130
1081
1067
nCO, rCH
rCH
nasCOC
3
159 sh
132 vs
097 m
079 w
033 vs
3
1130 vw
1020 vw
nCO
1069
1067
n
n
n
n
asCOC
asPOC
1051
1022
1
022
980
927
1022
978
926
1028
973
1023
973
1024
s
s
COC
COC
9
9
65 s
49 sh
n
asPOC
9
13 w
915
824
913
824
8
8
44 m
07 vw
852 w
849
849
735
nCO
8
00
800
766
7
92 vw
7
52 sh
730
575
n
s
POC
741 m
543 m
514 sh
494 m
457 s
743 m 0.5
550 w 0.6
720
759
711
570
dCCO
509
502
457
513
494
457
500 vs 0.15
452 m 0.8
505
457
452
407
497
461
437
503
457
425
399
496
455
424
n
n
s 2
PCl
asPCl
2
4
3
3
40 sh
97 m 0.36
75 vw
dCCO
dClPO
395
386
367
373
359
dCCO
3
2
30 vw
70 vw
329
348
321
3
15
285
13
206
294
269
214
200
307
233
213
204
302
267
215
204
wPCl
2
dCCO
tors
dPCl
2
251
223
196
186
155
2
219
211
2
1
1
10 s 0.6
95 sh
55 vw
147
161
160
153
tors

S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
21
Table 4
Continued
Observed (liquid)
Calculated for the conformers
IR
Raman
AAGA (0.1) GAGA (0.0) AAAA (0.7) GAAA (0.8) AGGЈA(1.0) GGGЈA (0.8) Assigment for
GGGЈA
−1
−1
−1
−1
−1
−1
−1
−1
n (cm ) I
n (cm ) I r
n (cm
)
n (cm
)
n (cm
)
n (cm
)
n (cm
)
n (cm )
1
40 vw 0.6
136
154
116
101
125
1
16
114
96
107
85
108
85
tPCl
tors
2
8
0 vw
95
87
Definitions as for Table 2.
due mainly to PCl stretching, while the latter bands
are associated with POCCC bending and PO stretching
extreme weakness of the bands at 560, 587 and
−1
−1
612 cm compared with the 496 cm band is indica-
tive of significant differences in DG between the anti
and gauche conformations about the O–C bond. Earlier
we observed an even stronger preference of anti con-
formations about the O–C bond in the case of the
(
Table 2). It appears, however, that the intensity of the
−1
band at 496 cm increases, relative to the intensities of
−1
the bands at 560, 587 and 612 cm upon cooling of
liquid 2, suggesting energetic preference of the
aforementioned anti conformations about the O–C
bond, in respect to gauche conformers. Besides, the
EtOPCl molecule [14].
2
These results agree well with molecular mechanics
4 9 2
Fig. 2. Temperature dependence of the infrared spectra of C H OPCl (3).

2
2
S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
data (Table 3), which suggest the energy of the gauche
conformations about the P–O and O–C bonds to be
higher than the energy of the corresponding anti
So, all aforementioned spectral measurements of DH
agree closely with computed conformational energy
data.
−1
conformations by ϳ0.2–0.6 kcal mol , and by
−1
ϳ0.6–1.5 kcal mol , respectively. Our computations
3.2. CH -O-CH -CH -O-PCl (3)
3
2
2
2
also predict that the energy of the conformers, absorb-
−
1
−1
ing at 1305 cm , should be about 1 kcal mol higher
than the energy of the conformers, giving rise to the
Just as in the case of molecule 2, the observed
vibrational spectra of the liquid 3 (Table 4, Fig. 2)
may be explained by the coexistence of several con-
formers. The molecular mechanics evaluations of the
energies of the latter are listed in Table 5.
−1
band at 1320 cm (Tables 2 and 3). According to the
temperature dependence of the intensity ratio of these
bands the enthalpy difference between these two groups
−1
−1
of conformations is equal to 0.9 Ϯ 0.3 kcal mol .
The Raman lines at 740 and 752 cm are assigned
Table 5
Energy and geometry characteristics of stable conformations of CH
3
-O-CH
2
-CH
2
2
-O-PCl (3), calculated by the molecular mechanics method
a
1
a
2
a
3
a
4
−1
No.
Name
f
(Њ)
f
(Њ)
f
(Њ)
f
(Њ)
U (kcal mol )
1
2
3
4
5
6
7
8
9
AAAA
AAAG
AAGA
AAGG
AAGGЈ
AGAA
AGAG
AGAGЈ
AGGA
AGGG
AGGGЈ
AGGЈA
AGGЈGЈ
GAAA
GAAG
GAAGЈ
GAGA
GAGG
GAGGЈ
GAGЈA
GAGЈG
GAGЈGЈ
GGAG
GGGA
GGGG
GGGGЈ
GGGЈA
GGGЈGЈ
GGЈAA
GGЈAG
GGЈAGЈ
GGЈGG
GGЈGЈA
GGЈGЈG
GGЈGЈGЈ
180
179
180
189
192
151
147
151
153
150
133
153
141
68
67
66
69
71
65
66
63
61
74
44
73
73
34
26
62
62
90
67
80
60
180
179
186
203
198
67
68
67
63
67
180
176
73
62
73
176
184
175
61
59
73
− 76
− 72
178
179
180
72
180
75
184
70
− 72
179
83
− 85
188
80
− 84
172
− 89
178
82
− 83
185
81
− 91
174
77
− 67
86
0.66
2.12
0.06
0.85
0.85
2.12
3.36
4.10
0.63
3.16
2.21
1.01
2.84
0.78
2.31
2.72
0
2.02
1.01
0.65
1.13
1.83
3.22
2.42
3.26
2.07
0.84
4.25
3.26
4.60
4.51
2.95
2.01
3.71
3.06
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
78
76
81
184
184
183
195
192
186
177
176
164
109
70
68
66
− 74
− 71
− 62
189
59
190
76
108
106
73
64
62
− 91
173
− 90
179
81
− 82
85
182
79
− 76
− 76
179
177
177
64
− 59
− 69
− 56
71
− 87
− 87
− 63
− 111
− 55
− 81
− 59
79
− 80
af
the phosphorus lone pair and the O–C bond (f
— dihedral angles of rotation around P–O, O–C3, C3–C4 and C4–O5 bonds. They are equal to zero in eclipsed position of
), P–O and C3–C4 bonds (f ), O–C and C4–O5 bonds (f ), C3–C4 and O5–C6 bonds (f ).
1
2 3 4
, f , f , f
1
2
3
4

S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
23
to the anti and gauche conformations about the P–O
the case of the molecule 2. The experimental value of
DH between the anti and gauche conformations about
the C3–O2 bond is equal to 0.8 Ϯ 0.2 kcal mol , the
bond, respectively, and the IR bands at 516 and
−1
−1
5
50 cm to the anti and gauche conformations
about the C–O bond (Table 4). From the
temperature dependence of the intensity ratio of
these bands in the temperature range from 35ЊC to
gauche conformer again being more stable.
These results do not agree with molecular
mechanics data (Table 5), according to which the
energy of the gauche conformations about the CO
and PO bonds is much higher than that of the corre-
sponding anti conformations. It is significant, that a
similar disagreement was noted in the case of mole-
cule 1 [3], but nothing of this kind was noted in the
−
5ЊC the enthalpy difference between the confor-
mations was obtained. Spectral measurements gave
−1
a value of 0.9 Ϯ 0.4 kcal mol for DH between the
anti and gauche conformations about the P–O bond,
the gauche conformer being more stable in contrast to
4 9 2 3 2 2 2
Fig. 3. (a) AAGG conformation of the C H OPCl molecule; (b) GGGЈA conformation of the CH OCH CH OPCl molecule.

2
4
S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
conformational behaviour of the molecule 2 (see pre-
vious subsection of this work). What the molecules 1
and 3 have in common, and what differentiates
them from the molecule 2, is the presence of both
electron-acceptor (P) and electron-donor (S or O)
heteroatoms in the first and fifth position, respec-
tively. In the case of 1 the disagreement was observed
1–13 (Table 5), i.e. i = 1, 2,…, 13. The second term
corresponds to the conformers no. 14–35 ( j = 14,
15,…, 35). The quantities E and E may be adequately
i
j
approximated by the calculated conformational
energies U and U from Table 5, except the case of
i
j
conformers with intramolecular O5…P1 attraction.
For the latter E should be replaced by (U − Eintra),
j
j
[
3] for convoluted conformations with close intra-
where Eintra is the energy of the intramolecular
O5…P1 attraction, neglected in our molecular
mechanics computations. According to the latter,
close intramolecular contacts between P1 and O5
atoms are possible in conformations no. 27, 28 and
32–35. But the attraction hardly could arise in the case
of j = 32, 33 and 34, because of unfavourable
mutual orientation of electron lone pairs of the P1
and O5 atoms. On this basis Eq. (1) may be rewritten
as
molecular 1–5 contact between the P and S atoms
of molecule 1. It was attributed to certain attractive
intramolecular interactions between the sulfur and the
phosphorus atoms which were, apparently, not taken
into account in the molecular mechanics calculations.
…
Most probably, such a kind of attractive 1–5 P
O
intramolecular interaction causes the energetical pre-
ferability of the gauche conformers of molecule 3,
contrary to what might be expected from molecular
mechanics considerations. According to our computa-
tional estimates, the distance between the O5 and P1
atoms (Fig. 3) is comparable to the sum of their van
der Waal’s radii in the case of the GGGЈA, GGGЈGЈ,
GGЈGG, GGЈGЈA, GGЈGЈG and GGЈGЈGЈ conforma-
tions. Thus, it is most likely that there arises weak
ꢀ
X
DH(PO) =
U W exp ( −U =RT)=
i i i
i = 1, 2, …, 13
ꢁ
X
W exp ( −U =RT)
i
i
…
ꢀ
attractive P1 O5 interaction, leading to the energetic
X
stabilization of the gauche conformers of molecule 3.
A comparison of the molecular mechanics evalua-
tion of the relative energy of the gauche and anti
conformations about the PO and CO bonds (Table
−
UjWj exp ( −Uj=RT)=
j = 14, 15, …, 26, 29–34
ꢁ
X
W exp ( −U =RT)
j
j
5
) with the aforementioned experimental DH values
ꢂ
ꢃ
X
…
allows to estimate the energy of the O5 P1 interac-
tions, neglected in our computations. The experimen-
tally determined DH(PO) value is the difference
between the weight-average energies of all the anti
conformations about the P–O bond and all the gauche
conformations about the P–O bond. This may be writ-
ten analytically in the following form [29,30]:
−
(U − E )W exp [ −(U − E )=RT] =
j
intra
j
j
intra
j = 27, 28, 35
X
Wj exp [ −(Uj − Eintra)=RT]
ꢀ2†
For the sake of simplicity, let us assume that the
desired values Eintra are the same for the conformers
no. 27, 28 and 35. Then, using the experimental value
of DH(PO), we will find Eintra = 2.0 kcal mol . Simi-
lar procedure for DH(OC) gives Eintra = 2.4 kcal mol .
DH(PO)
−1
ꢀ
ꢁ
−1
X
X
X
=
E W exp ( −E =RT)=
W exp ( −E =RT)
i i
i
i
i
So, both these estimates are sufficiently consistent.
In the case of molecule 1 Eintra was estimated [3]
only for the GGЈGЈGЈ conformation, but not for the
GGGЈA and GGGЈGЈ conformations, because the
latter did not correspond to minima on the potential
i
anti about P–O
ꢀ
ꢁ
1†
X
−
E W exp ( −E =RT)=
W exp ( −E =RT)
j j
j
j
j
j
gauche about P–O
…
energy surface of internal rotation. E (P S)
intra
−1
ꢀ
averaged between ϳ2.5 and ϳ4 kcal mol [3]. If
In the first term of the Eq. (1) E and W represent the
energies and statistical weights of the conformers no.
we suppose that in molecule 3 the intramolecular
i
i
…
P O attraction also could arise only in GGЈGЈGЈ

S. A. Katsyuba et al./Journal of Molecular Structure 475 (1999) 13–25
25
(
no. 35) conformation, then we would obtain from the
References
−1
…
Eq. (1) Eintra(P O) Ϸ 4.5 kcal mol . So, within the
…
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…
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(
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and molecular mechanics computations for the mole-
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1
[
[
…
…
lecular 1–5 P S and P O attraction lies in the range
−1
between ϳ2 and ϳ4.5 kcal mol . It would be inter-
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This paper was prepared with support of the Rus-
sian Foundation of Academic Research (RFAR),
Grant no.97-03-33691a. The authors also gratefully
acknowledge Dr S. A. Terent’yeva for supplying
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