Experimental and quantum chemical evidences for C-H···N hydrogen bonds involving quaternary pyridinium salts and pyridinium ylides

Article abstract of DOI:10.1016/S0022-2860(00)00585-8

The rates of equilibrium of eight quaternary pyridinium salts containing activated methylene groups with the corresponding ylides have been measured in DMSO solution containing strong bases (DBU, MTBD and P1-tBu). The effect of the ring substituents on th

Full text of DOI:10.1016/S0022-2860(00)00585-8

Journal of Molecular Structure 555 (2000) 31–42
www.elsevier.nl/locate/molstruc
Experimental and quantum chemical evidences for C–H···N
hydrogen bonds involving quaternary pyridinium salts and
pyridinium ylides
*
Z. Dega-Szafran , G. Schroeder, M. Szafran, A. Szwajca, B. Łe¸ska, M. Lewandowska
Faculty of Chemistry, Adam Mickiewicz University, ul Grunwaldzka 6, 60-780 Poznan, Poland
Received 16 November 1999; revised 5 January 2000; accepted 5 January 2000
Abstract
The rates of equilibrium of eight quaternary pyridinium salts containing activated methylene groups with the corresponding
ylides have been measured in DMSO solution containing strong bases (DBU, MTBD and P1-tBu). The effect of the ring
substituents on the deprotonation rate is greater than that of the methylene substituents. This observation is consistent with the
energy of C–H···N hydrogen bonds and the elongation of the C–H bond participating in hydrogen bonds estimated by the PM3
1
and BLYP/6-31G(d,p) calculations. The UV spectra and H NMR spectra of selected ylides have been measured. The solvent
effect on the proton chemical shifts of quaternary pyridinium halides and ylides is discussed. Semiempirical and DFT calcula-
tions were carried out to investigate the structure of ylides. In ylides, the a-pyridine protons are more acidic than the methine
proton. ᭧ 2000 Elsevier Science B.V. All rights reserved.
Keywords: N-quaternary cations; Ylides; Deprotonation; C–H···N hydrogen bond
1. Introduction
heterocyclic ring may occur and the stability of such
compounds is enhanced. Pyridine ylides with CyO or
CN groups are so stable that their structures were deter-
mined by X-ray diffraction [6,9–13].
In quaternary ammonium halides of the type ϾNϩ–
CH2–Y, where Y is a strongly electron-withdrawing
substituent, the methylene protons are acidic and their
reactivity becomes comparable with that of the methy-
lene group of b-ketoesters [1]. Strong bases can abstract
one of such protons which may lead to the formation of
ylides [2–7]. Most N-ylides are capable of only fleeting
existence, but they may be stabilized by coordination
with lithium halides [8]. On the other hand, in pyridi-
nium-type ylides, a delocalization of charge into the
Ammonium ylides are important precursors in
organic synthesis, due to their Stevens (1,2) and
Sommelet-Hauser (2,3) rearrangement [14] under
mild conditions to form highly substituted organic
compounds and the possibility to prepare from them
stereospecific compounds [15]. Heterocyclic N-ylides
with five and six-membered rings are known as a
mesoionic compounds and one of their most inter-
esting properties is that they behave as 1,3-dipoles
and undergo 1,3-dipolar additions with a great number
of dipolarophiles (also known as 3 ϩ 2 cycloaddition
reactions) [2–7,16]. Another feature of N-ylides is
* Corresponding author. Tel.: ϩ48-61-829-1216; fax: ϩ48-61-
865-8008.
E-mail address: szafran@main.amu.edu.pl (Z. Dega-Szafran).
0022-2860/00/$ - see front matter ᭧ 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(00)00585-8

32
Z. Dega-Szafran et al. / Journal of Molecular Structure 555 (2000) 31–42
Scheme 1.
their conversion to olefines or less substituted amines
[17–22].
stand for 1–3 weeks at room temperature, until the
solid was precipitated. The mixture containing
BrCH2COPh required refluxing for 10 h. The excess
of the solvent was removed under reduced pressure,
the crude product was washed with diethyl ether and
recrystallized (Table 1). The CH2–Nϩ protons
The ability of carbon atoms to act as proton donors
in hydrogen bonds has been the subject of controversy
for many years [23,24]. Spectroscopic studies [25]
indicate that C–H···O and C–H···N occur in many
systems, a conclusion that is supported by quantum
chemical calculations [26–30].
1
exchange slowly with D2O and in H NMR spectrum
the signal disappears.
In this paper, we investigate the deprotonation
reactions of eight quaternary pyridinium halides,
shown in Scheme 1, by strong bases in DMSO and
perform semiempirical and DFT calculations in order
to get information on the C–H···N hydrogen bonds.
1-(Phenacyl)pyridinium ylide (3d) was prepared by
a slightly modified procedure described in Ref. [1]. A
solution of potassium carbonate (3.5 g) in water
(3 ml) was added with cooling to a solution of N-(4-
methylpyridinium)phenacyl bromide (2 g) in water
(4 ml). The mixture was stirred for 20 min and then
extracted with dichloromethane. The extract was
dried for 1 h with Na2SO4 and solvent was evaporated
in vacuum. The red gun crystallized to orange
crystals, m.p. 104–106ЊC.
Bases: 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
ꢀpKaAN ˆ 23:9 [35]; PA ˆ 1047:9 kJ=mol [36])
7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD)
ꢀpKaAN ˆ 24:70 [37]; PA ˆ 1062:7 kJ=mol [32]); and
2. Experimental
Quaternary pyridinium halides were prepared
by treatment of the appropriate substituted pyridine
(0.1 M) with the equimolar amounts of ClCH2COOEt,
ClCH2CONH2, ClCH2CN, ClCH2COMe, or BrCH2
COPh, respectively, in acetonitrile (AN) or acetone
(30 cm3) (Scheme 1). The mixture was allowed to
Table 1
Reaction of R-pyridines with X–CH2–Y
Comp.
Reaction conditions
Solvent for crystallization
M.p. (ЊC)
Yield crude (%)
1a
1b
1c
1d
2a
2b
2c
2d
2e
MeCOMe, 25Њ, 3 days
25Њ, 3 days
(Et)2O, reflux, 20 h
MeCN
MeCN:MeOH (2:1)
MeCN
EtOH
MeCN
MeCN:MeOH (2:1)
MeCN
MeCN:MeOH (2:1)
MeCN:MeOH (2:1)
226–227a
157b
75
93
65
99
77
39
17
62
17
64–67
Steam-bath, 30 min
245c
C6H6:MeCN (5:9), 25Њ, 4 days
(Et)2O:C6H6 (9:1), rt, 4 days
MeCOMe, 25Њ, 4 days
MeCOMe, reflux, 12 h
MeCOMe, 25Њ, 2 weeks
295–296
202–203
258–259
228–229d
229–230
a
M.p. 230–232ЊC [31].
M.p. 151ЊC [32].
M.p. 274–276ЊC [33].
M.p. 215–217ЊC [34].
b
c
d

Z. Dega-Szafran et al. / Journal of Molecular Structure 555 (2000) 31–42
33
Table 2
1H NMR data (ppm) of the investigated compounds (s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; b, broad)
Comp.
Solvent
Ha
Hb
PyϩCH2
R
Y
1a
D2O
8.41(2H, d)
8.80(2H, d)
8.46(2H, d)
8.44(2H, d)
8.94(2H, d)
8.92(2H, d)
8.27(2H, d)
8.85(2H, d)
8.88(2H, d)
8.37(2H, d)
8.90(2H, d)
8.59(2H, d)
9.09(2H, d)
7.75(2H, d)
8.18(2H, d)
7.76(2H, d)
8.32(2H, d)
7.61(2H, d)
8.16(2H, d)
7.82(2H, d)
8.24(2H, d)
7.88(2H, d)
8.41(2H, d)
9.58 (2H,d)
9.50 (2H,d)
7.73(2H, d)
7.99(2H, d)
7.84(2H, d)
7.74(2H, d)
8.07(2H, d)
7.88(2H, d)
7.73(2H, d)
8.06(2H, d)
7.87(2H, d)
7.81(2H, d)
8.12(2H, d)
7.91(2H, d)
7.82(2H, d)
6.71(2H, d)
7.03(2H, d)
6.72(2H, d)
7.12(2H, d)
6.70(2H, d)
7.09(2H, d)
6.73(2H, d)
7.15(2H, d)
6.76(2H, d)
7.14(2H, d)
7.35(2H, b)
7.40(2H, d)
5.23(2H, s)
5.33(2H, s)
5.25(2H, s)
5.29(2H, s)
5.66(2H, s)
5.72(2H, s)
5.50(2H, s)
5.90(2H, s)
6.06(2H, s)
6.14(2H, s)
6.50(2H, s)
6.31(2H, s)
7.09(2H, s)
4.80(2H, s)
4.94(2H, s)
4.85(2H. s)
5.29(2H. s)
5.04(2H. s)
5.40(2H. s)
5.67(2H. s)
6.04(2H, s)
5.14(2H, s)
5.58(2H, s)
6.79(1H, b)a
6.71(1H, b)a
2.48(3H, s)
2.63(3H, s)
2.65(3H, s)
2.49(3H, s)
2.65(3H, s)
2.65(3H, s)
2.48(3H, s)
2.64(3H, s)
2.63(3H, s)
2.52(3H, s)
2.68(3H, s)
2.69(3H, s)
2.74(3H, s)
3.02(6H, s)
3.20(6H, s)
3.03(6H, s)
3.22(6H, s)
3.02(6H, s)
3.21(6H, s)
3.03(6H, s)
3.24(6H, s)
3.05(6H, s)
3.23(6H, s)
2.40(3H, s)
2.43(3H, s)
–
DMSO-d6
CD3CN
D2O
DMSO-d6
CD3CN
D2O
DMSO-d6
CD3CN
D2O
DMSO-d6
CD3CN
CDCl3
D2O
DMSO-d6
D2O
DMSO-d6
D2O
DMSO-d6
D2O
8.08(1H, b), 7.69(1H, b)
–
4.11(2H, q), 1.09(3H, t)
4.23(2H, q), 1.26(3H, t)
4.26(2H, q), 1.28(3H, t)
2.22(3H, s)
1b
1c
1d
2.31(3H, s)
2.31(3H, s)
7.87(2Ho, d), 7.43(2Hm, t), 7.60 (1Hp, t)
8.09(2Ho, d), 7.67(2Hm, t), 7.81 (1Hp, t)
8.08(2Ho, d), 7.64(2Hm, t), 7.78 (1Hp, t)
8.21(2Ho, d), 7.57(2Hm, t), 7.69 (1Hp, t)
–
7.80(1H, b), 7.51(1H, b)
4.09(2H, q), 1.09(3H, t)
4.20(2H, q), 1.24(3H, t)
2.14(3H, s)
2a
2b
2c
2d
2e
3d
2.23(3H, s)
7.70(2Ho, d), 7.40(2Hm, t), 7.56 (1Hp, t)
8.05(2Ho, d), 7.65(2Hm, t), 7.78 (1Hp, t)
–
DMSO-d6
D2O
DMSO-d6
CD3CN
CDCl3
–
7.78(2Ho, d), 7.35(2Hm,p, m)
7.83(2Ho, d), 7.35(3Hm,p, m)
a
dCHϪ.
Schwesinger base, phosphazene (P1-tBu) ꢀpKaAN
ˆ
The PM3 [39], AM1 [40] and SAM1 [41] semiem-
pirical calculations were performed using the ampac 6.0
program [42]. In all cases, the PRECISE and GRAD
keywords were used and the full geometry optimisation
was carried out without any symmetry constraints. The
ab initio calculations were run with the gaussian 98
[43] program packages. The molecular parameters of
the complex were determined at the DFT [44] level
using the standard 6-31 G(d,p) basis set [45] with two
different functionals BLYP and B3LYP (the exchange
functional of Becke (B) [46] and the gradient-corrected
functional of Lee et al. (LYP) [47].
33:49; pKaDMSO ˆ 21:5 [38]; PA ϳ 1100 kJ/mol [36])
were commercial materials purchased from Fluka and
were used as received.
1
The H NMR spectra were recorded on a Varian
Gemini 300VT spectrometer, operating at
300.07 MHz, using TMS as the internal standard.
2D (H,H-COSY) and homodecoupling methods
were used to verify proton chemical shifts in the
investigated compounds. The UV spectroscopy was
recorded in DMSO on a Diode Array Hewlett-Packard
spectrophotometer.
The rates of proton transfer were measured spectro-
photometrically in a Applied Photophysics stopped-
flow apparatus with the cell block thermostated to
^1ЊC. The reactions were monitored at maximum
absorption of ylide (400–497 nm). The reactions
were conducted by mixing a DMSO solution of 1a–
e–2a–e with the base in DMSO. The concentration of
1 and 2 was 2 × 10Ϫ4 M.
3. Results and discussion
1
3.1. H NMR and UV–Vis spectra of quaternary
pyridinium halides and pyridinium ylides
Pyridines have displayed pronounced substituent

34
Z. Dega-Szafran et al. / Journal of Molecular Structure 555 (2000) 31–42
1
Fig. 1. H NMR spectrum of 1d and 3d in CDCl3.
and solvent effects [48,49]. Conversion of pyridine
into pyridinium cation causes a downfield shift of
all the hydrogens [50]. Table 2 summarises the
chemical shifts of the protons in the investigated
salts in several solvents. As expected, the chemical
shifts are affected by substituents and solvents. The
observed solvent effect on the chemical shifts (Table
2) may be explicable in terms of variation of ion
pairing between the pyridinium ion and counterion
and changes of the resonance hybrid. The chemical
shifts of the pyridine ring protons in DMSO-d6 solu-
tion of 4-NMe2-pyridinium salts are comparable to
these 4-NMe2-pyridinium methiodide [51].
Fig. 1 shows spectra of 1d and 3d in CDCl3.
Conversion of the quaternary salt into the ylide causes
a downfield shift of a-pyridine proton and an upfield
shift of b-pyridine and CH3 protons. Similar changes
of chemical shifts have been observed by Phillips and
Ratts [52] and by Matsuboyashi [53]. The methine
proton appears as a broad signal at 6.71 ppm. All
signals of phenyl protons are deshielding (compare
values for 1d and 3d in Table 2). It is interesting to
note that solvent has an influence on the multiplicity
of the ylide protons. In CD3CN the pyridine ring
protons appeared as broad singlets, whilst in CDCl3
solution they were doublets (Fig. 1). These differences

Z. Dega-Szafran et al. / Journal of Molecular Structure 555 (2000) 31–42
35
decreasing of the 442 nm band (Fig. 2) reflects
decomposition of the ylide.
3.2. Kinetics of proton transfer to amine bases
When quaternary pyridinium halide (AH) is treated
by an excess of a strong base (B) {‰BoŠ q ‰AHoŠ} the
following processes can occur:
k
ϩ
AH ϩ B O ꢀAϩ=Ϫ H–B †; kobs ˆ k‰BoŠ ϩ kϪ ꢀ1†
…
kϪ
k
AH ϩ B O Aϩ=Ϫ ϩ BϩH; kobs ˆ k‰BoŠ ϩ kϪ‰BϩHŠ
kϪ
ꢀ2†
Fig. 2. UV–Vis spectra of 1c ꢀ2 × 10Ϫ4M† and a mixture of 1c ꢀ2 ×
10Ϫ4M† with MTBD ꢀ2 × 10Ϫ3M† in DMSO; curve 1, 0 min; 2,
10 min; 3, 20 min; 4, 30 min; 5, 40 min; 6, 50 min; 7, 60 min; 8,
70 min.
kD
k
ϩ
AH ϩ B O ꢀAϩ=Ϫ H–B † O Aϩ=Ϫ ϩ BϩH;
…
kϪ
ꢀ3†
kobs ˆ k‰BoŠ ϩ kϪ‰BϩHŠ=KD
may be explained in terms of dynamic aspects of
solvation. CDCl3 as weak proton donor can form
weak hydrogen bonds with the oxygen atom, elec-
tron pair of methine group and p-electrons of
pyridine and phenyl rings. CD3CN with ylides
can form a dipole–dipole complex. The NMR
data indicate that the life-time of solvated species
in CD3CN is comparable with the time of obser-
vation, while in CDCl3 it is longer [54]. Another
explanation is given by entropy [55]. The NMR
spectra reflect variation of structure and dynamics
of solvated species.
Fig. 2 shows the UV–Vis spectrum of 1c in DMSO
and its time-dependent changes on addition of MTBD.
The absorption in the 422–497 nm region (Table 3;
Fig. 2) is due to an intramolecular charge-transfer
transition [1,53,56]. The significant intensity
where k is the rate constant for the deprotonation
(forward) reaction, kϪ is the rate constant for the
protonation (backward) reaction, [Bo] is the initial
base concentration, [BϩH] is the concentration of
the cation and KD is the dissociation constant of the
ion pair (AϪ···H–Bϩ). The kobs values were calculated
from the traces of absorbance (400–497 nm) vs. time
and are listed in Table 4. When kobs was plotted vs.
[Bo], excellent straight lines were obtained (r Ͼ 0.99),
where the slope gives k values and the intercept kϪ
values (Table 4), which means that the reaction
studied was correctly described by Eq. (1). Note that
in the case of Eq. (2), term [BϩH] is not constant; for
the forward reaction the process is of the first order,
but for the backforward reaction is of the second
order.
Although Eq. (1) correctly fitted the experimental
data, model (3) cannot be completely neglected. The
dissociation of the hydrogen bonded ion pairs into
ions increases with dilution and increasing tempera-
ture. It seems that the contribution of solvated ions can
be neglected in the range of concentration and
temperature applied in this work.
Table 3
UV–Vis absorption spectra of 4-R–C5H4Nϩ–CϪH–Y in DMSO-d6
Ylides
l (nm)
log e
3a
3b
3c
3d
4b
4c
446
422
424
442
414
410
3.06
3.78
4.48
3.87
3.15
3.33
The rate of deprotonation, as expected, is primary
affected by the substituent in the pyridine ring. All
salts with 4-NMe2 substituent show ca 4–6 order of
magnitude slower reactivity than the corresponding
salts with 4-Me group. The unsubstituted pyridine

36
Z. Dega-Szafran et al. / Journal of Molecular Structure 555 (2000) 31–42
Table 4
Kinetic data (standard deviation) for the deprotonation of 4-R–C5H4Nϩ–CH2–Y XϪ
Comp.
Base
Bo (M)
kobs (sϪ1
30ЊC
)
35ЊC
40ЊC
45ЊC
50ЊC
1a
1b
MTBD
DBU
0.05
0.1
140(20)
260(20)
330(20)
417(6)
580(5)
760(2)
147(2)
198(3)
260(1)
358(2)
518(1)
695(3)
123(1)
144(2)
177(3)
210(10)
290(10)
372(20)
190(10)
350(20)
420(20)
529(6)
750(6)
1030(3)
174(3)
230(4)
298(2)
480(3)
690(3)
905(2)
140(3)
166(4)
207(5)
300(10)
400(10)
510(10)
240(30)
420(30)
510(20)
640(8)
920(7)
1303(4)
200(4)
259(5)
333(3)
580(7)
800(5)
1060(7)
162(4)
189(3)
242(5)
380(10)
490(10)
660(20)
300(20)
480(30)
620(20)
750(9)
1080(8)
1600(6)
240(1)
249(5)
372(1)
700(8)
950(10)
1250(12)
180(5)
230(7)
275(6)
490(10)
670(20)
910(20)
0.125
0.015
0.03
0.05
0.015
0.03
0.05
0.0015
0.003
0.005
0.0015
0.003
0.005
0.0015
0.003
0.005
0.1
MTBD
DBU
1c
MTBD
MTBD
DBU
1d
2b
0.00055
0.00173
0.00295
0.00066
0.00161
0.00227
0.0129
0.00082
0.00157
0.00281
0.00381
0.00827
0.01210
0.00413
0.00697
0.01010
0.0532
0.2
0.3
0.00263
0.00413
0.00126
0.00252
0.00368
0.0276
0.00428
0.00651
0.00208
0.00383
0.00561
0.0430
0.00613
0.00924
0.00288
0.00532
0.00811
0.0429
MTBD
P1-tBu
DBU
0.05
0.075
0.1
0.005
0.015
0.025
0.1
0.0295
0.0466
0.0551
0.0832
0.0880
0.1300
0.1260
0.1900
0.1610
0.2560
2c
2d
2e
0.00221
0.00266
0.00291
0.00339
0.00423
0.00511
0.00590
0.00625
0.00685
0.00116
0.00148
0.00165
0.00150
0.00156
0.00163
0.00137
0.00155
0.00168
0.00220
0.00260
0.00290
0.00320
0.00380
0.00268
0.00324
0.00375
0.00509
0.00644
0.00793
0.00680
0.00718
0.00805
0.00145
0.00184
0.00216
0.00179
0.00192
0.00199
0.00169
0.00195
0.00214
0.00240
0.00280
0.00317
0.00360
0.00542
0.00296
0.00361
0.00435
0.00642
0.00813
0.00971
0.00785
0.00872
0.00977
0.00172
0.00226
0.00271
0.00210
0.00234
0.00243
0.00198
0.00221
0.00270
0.00255
0.00304
0.00355
0.00398
0.00830
0.00322
0.00410
0.00499
0.00732
0.00971
0.01150
0.00891
0.00992
0.01140
0.00194
0.00258
0.00316
0.00268
0.00294
0.00313
0.00250
0.00294
0.00342
0.00274
0.00340
0.00387
0.00425
0.01030
0.00343
0.00444
0.00562
0.00833
0.01130
0.01430
0.01010
0.01140
0.01280
0.00235
0.00314
0.00385
0.00299
0.00335
0.00365
0.00286
0.00344
0.00401
0.00288
0.00360
0.00410
0.00452
0.01200
0.2
0.3
MTBD
P1-tBu
DBU
0.050
0.075
0.100
0.005
0.015
0.025
0.1
0.2
0.3
MTBD
P1-tBu
DBU
0.05
0.075
0.1
0.005
0.015
0.025
0.05
0.1
0.15
0.2
0.05
MTBD

Z. Dega-Szafran et al. / Journal of Molecular Structure 555 (2000) 31–42
37
Table 4 (continued)
Comp.
Base
Bo (M)
kobs (sϪ1
30ЊC
)
35ЊC
40ЊC
45ЊC
50ЊC
0.1
0.15
0.2
0.025
0.035
0.05
0.065
0.00577
0.00842
0.01030
0.129
0.142
0.160
0.00850
0.01170
0.01450
0.161
0.178
0.203
0.01170
0.01520
0.01770
0.196
0.217
0.254
0.01410
0.01730
0.02130
0.223
0.265
0.319
0.01610
0.02050
0.02500
P1-tBu
0.177
0.234
0.299
0.369
salts react ca 90-fold faster than 4-Me-pyridine salts
[57]. In the case of 4-CN-pyridine salts, deprotonation
was too fast to be measured in our stopped-flow appa-
ratus. These data indicate that electron-withdrawing
substituents accelerate deprotonation, while electron-
donating ones decelerate it down. Similar effect of
substituents on the reaction of abstraction of b-proton
from N-(2-cyanoethyl) pyridinium cations in aqueous
solution was observed by Bunting et al. [21]. The effect
of methylene substituents on the rate of
proton abstraction is much weaker than that of
ring substituents. Abstraction of proton by MTBD in
4-Me–C5H4Nϩ–CH2COMe is ca 20-fold faster than in
4-Me–C5H4Nϩ–CH2CONH2. In the case of 4-NMe2–
C5H4Nϩ–CH2Y the difference between the fastest and
the slowest reaction is 12 and 140-fold, respectively, in
MTBD and P1-tBu. The effect of the base strength
varies with the type of salt. In 4-NMe2–C5H4Nϩ–
CH2Y, the rate of proton abstraction, as expected,
increases with increasing pKa value of the super
base applied. However, this trend is broken in the
case of 4-Me–C5H4Nϩ–CH2Y salts.
The activation parameters (Table 5) were calcu-
lated from the Eyring equation in form (4) [58,59]:
k
kB
DS±
DH±
log ˆ log
ϩ
Ϫ
ϩ
T
ប
2:303R
2:303RT
DS±
1000DH±
19:137T
ˆ 10:319 ϩ
ꢀ4†
19:137
Table 5
Activation parameters for proton abstraction from 4-R–C5H4Nϩ–CH2–Y XϪ in DMSO at 30ЊC
Comp.
Base
k (l molϪ1 sϪ1
)
Int (sϪ1
)
DH± (kJ molϪ1
)
DS± (J molϪ1 degϪ1
)
DG± (kJ molϪ1
)
1a
1b
MTBD
DBU
MTBD
DBU
MTBD
MTBD
DBU
MTBD
P1-tBu
DBU
MTBD
P1-tBu
DBU
MTBD
P1-tBu
DBU
2530 (180)
9760 (520)
3220 (80)
95870 (5100)
15490 (700)
45620(1420)
0.0120 (1)
0.032 (3)
13 (7)
24.5(0.8)
46.1(2.9)
6.19(2.5)
23.1(2.5)
26.9(1.3)
47.3(2.3)
48.7(2.2)
53.0(4.3)
71.9(2.5)
42.9(3.3)
44.2(4.7)
43.1(4.6)
42.8(3.2)
63.7(2.4)
51.9(4.2)
17.4(2.9)
22.2(3.6)
57.0(2.1)
Ϫ99
Ϫ17
Ϫ157
Ϫ73
54.5(0.8)
51.3(2.9)
53.8(2.5)
45.2(2.5)
49.9(1.3)
47.2(2.3)
85.4(2.2)
83.0(4.3)
72.8(2.5)
84.4(3.3)
82.7(4.7)
81.9(4.6)
89.5(3.2)
89.2(2.4)
84.6(4.2)
86.8(2.9)
82.2(3.6)
72.4(2.1)
277 (18)
100 (3)
220 (18)
99 (2)
144 (4)
ϳ0
1c
Ϫ76
1d
2b
0.2
Ϫ121
Ϫ99
ϳ0
0.0044(2)
1.69 (1)
Ϫ3
2c
2d
2e
0.0035 (6)
0.0344 (5)
0.048 (7)
0.0025 (4)
0.0026 (1)
0.0155 (14)
0.0066 (3)
0.044 (2)
0.0019(1)
0.0017(1)
0.0056(1)
0.0009(1)
0.0014(1)
0.0013(1)
0.0019(1)
0.0015(3)
0.010(1)
Ϫ150
Ϫ127
Ϫ128
Ϫ154
Ϫ84
Ϫ108
Ϫ229
Ϫ198
Ϫ55
MTBD
P1-tBu
1.197 (23)

38
Z. Dega-Szafran et al. / Journal of Molecular Structure 555 (2000) 31–42
Scheme 2.
Table 6
˚
Heats of formation (kcal), dipole moments (Debye), hydrogen bond energies (kcal) and selected distances (A) calculated by the PM3 method of
compounds from Scheme 2 (heats of formation for NH2Ϫ, 38.3; NH3, Ϫ3.1; NH4ϩ, 153.4; C5H5N, 30.4; C5H5NϩH, 187.3; 4-NH2–C5H5N, 27.5; 4-
NH2–C5H5NϩH, 175.2 kcal)
Comp. DH
m
EHB
C···Na (C)H···N C···H(N) C–Hb C–Hc
Nϩ···O C8–O Nϩ–C7 C7–C8 C2–Nϩ–C7–C8
Complexes
7ak
8ak
9ak
7am
8am
7an
8an
7bk
8bk
9bk
7ck
8ck
9ck
7dk
8dk
9dk
131.4
5.0 Ϫ10.1 2.914 1.748
8.8 Ϫ16.3 2.702
1.169
1.111 2.767
1.107 2.893
1.213 1.481
1.220 1.430
1.234 1.365
1.213 1.481
1.221 1.428
1.214 1.481
1.228 1.414
1.213 1.481
1.223 1.422
1.231 1.357
1.213 1.482
1.227 1.431
1.234 1.367
1.214 1.482
1.227 1.444
1.236 1.371
1.527 Ϫ86.0
1.484 Ϫ52.3
137.5
Ϫ4.01
162.3
175.7
157.2
165.1
174.9
178.2
31.2
119.7
125.4
Ϫ13.6
117.9
127.4
Ϫ7.1
1.605
4.4
Ϫ1.3 3.753 2.506
(1.123) 1.098 2.893
1.446
Ϫ0.0
5.3 Ϫ12.7 2.917 1.752
5.8 Ϫ12.0 2.725
5.5 Ϫ14.9 2.911 1.741
8.0 Ϫ10.5 2.752
6.8 Ϫ10.5 2.912 1.745
12.4 Ϫ11.2 2.729
1.167
1.170
1.170
1.111 2.767
1.106 2.911
1.111 2.762
1.103 2.888
1.111 2.762
1.105 2.886
1.526 Ϫ84.0
1.483 Ϫ51.5
1.525 Ϫ86.4
1.469 Ϫ21.4
1.528 Ϫ86.1
1.478 Ϫ24.0
1.579
1.676
1.648
5.3
4.8
Ϫ1.7 3.520 2.423
Ϫ9.8 2.916 1.740
(1.124) 1.098 2.896
1.167
1.453
Ϫ0.1
1.111 2.767
1.105 2.889
1.526 Ϫ86.0
9.0 Ϫ19.0 2.710
1.619
1.583
1.477
1.445
23.9
0.0
4.3
4.6
Ϫ1.5 3.575 2.507
Ϫ8.3 2.927 1.770
(1.104) (1.124) 2.897
1.160
1.110 2.779
1.105 2.889
1.524 Ϫ89.8
1.477 Ϫ24.7
8.4 Ϫ23.5 2.693
4.5
Ϫ1.5 3.598 2.505
(1.125) 1.097 2.896
1.441
Ϫ0.2
Ylides
6a
6b
6c
6d
0.39
36.0
Ϫ9.0
Ϫ2.5
3.4
4.0
3.6
4.3
1.097 2.856
1.098 2.894
1.097 2.895
1.096 2.894
1.233 1.364
1.231 1.357
1.234 1.365
1.235 1.369
1.448
1.455
1.447
1.444
0.0
0.0
0.0
0.0
Cations
5a
5b
5c
5d
144.6
188.5
132.6
129.3
4.4
5. 9
4.1
3.9
1.113 2.768
1.113 2.765
1.113 2.771
1.210 1.489
1.211 1.489
1.211 1.489
1.212 1.487
1.538 Ϫ89.4
1.539 Ϫ89.1
1.537 Ϫ89.0
1.533 Ϫ88.7
1.112
2.783
a
Distance between C(7)···N bond and between C(2)···N in parenthesis.
Length of C(7)–H (C(2)–H) bond engaged in hydrogen bond.
Length of C(7)–H bond non-engaged in hydrogen bond and in parenthesis C(6)–H bond.
b
c

Table 7
˚
Energies (a.u), dipole moments (Debye), hydrogen bond energies (kcal/mol) and selected distances (A) calculated by the BLYP/6-31G(d,p) method of compounds from Scheme 2
(energy of NH3, Ϫ56.5282252 and NH4Ϫ Ϫ56.8753721 a.u.)
Comp. Energy
m
EHB
C···Na (C)H···N C–Hb
C–Hc
Nϩ···O C8–O Nϩ–C7 C7–C8 C2–Nϩ–C7–C8 qNϩ
qO
qC
Complexes
7ak
7akd
9ak
7bk
7bkd
9bk
7dk
Ϫ497.0016536 3.94 Ϫ12.87
Ϫ497.0015812 4.00 Ϫ12.87
3.220
3.211
(3.463) (2.969)
3.193
3.194
2.104
2.092
1.121
1.121
1.098 2.762
1.098 2.763
1.224 1.482
1.224 1.482
1.265 1.396
1.223 1.479
1.223 1.479
1.261 1.387
1.225 1.479
1.563
1.563
1.426
1.567
1.567
1.435
1.557
Ϫ65.1
Ϫ65.5
Ϫ0.0
Ϫ63.5
Ϫ64.1
Ϫ0.0
Ϫ0.3311 Ϫ0.3737 Ϫ0.1940
Ϫ0.3310 Ϫ0.3740 Ϫ0.1956
Ϫ496.5854357 6.3
Ϫ3.2
(1.094) 1.091 2.978
1.124
1.123
(1.095) 1.087 2.971
1.116
Ϫ0.3603 Ϫ0.5290
0.0405
Ϫ589.214939
Ϫ589.214981
7.16 Ϫ12.94
7.12 Ϫ12.95
2.070
2.074
1.098 2.761
1.098 2.760
Ϫ0.3345 Ϫ0.3692 Ϫ0.1970
Ϫ0.3347 Ϫ0.3694 Ϫ0.1964
Ϫ0.3651 Ϫ0.5070 Ϫ0.0226
Ϫ0.3510 Ϫ0.3836 Ϫ0.1868
Ϫ588.8190129 7.40
Ϫ5.96 (3.412) (2.317)
Ϫ630.931716
3.77 Ϫ10.16 3.283 2.172
1.099 2.760
Ϫ69.0
Ylides
6a
Ϫ440.0491485 4.31
Ϫ532.2812866 4.53
Ϫ573.9548516 7.55
1.089 2.976
(1.089)
1.089 2.968
(1.089)
1.088 2.969
(1.089)
1.262 1.392
1.259 1.383
1.270 1.406
1.431
1.439
1.420
0.0
Ϫ0.2
0.0
Ϫ0.3561 Ϫ0.5094 Ϫ0.0186
Ϫ0.3612 Ϫ0.4902 Ϫ0.0031
Ϫ0.3634 Ϫ0.5446 Ϫ0.0442
6b
6d
Cations
5a
Ϫ440.4528046 3.60
1.100 2.713
(1.089)
1.098 2.758
1.102
(1.089)
1.099 2.765
1.103
1.220 1.485
1.221 1.482
1.578
1.581
Ϫ85.8
Ϫ0.3236 Ϫ0.3600 Ϫ0.1722
Ϫ0.3408 Ϫ0.3554 Ϫ0.1700
5b
5d
Ϫ532.66601
6.40
62.2
Ϫ574.3872828 3.13
1.223 1.478
1.568 Ϫ64,8
Ϫ0.3571 Ϫ0.3707 Ϫ0.1590
(1.088)
a
Distance between C(7)···NH3 and between C(2)···NH3 in parenthesis.
Length of C(7)–H (C(2)–H) bond engaged in hydrogen bond.
Length of C(7)–H (C(6)–H) bond non-engaged in hydrogen bond
b
c
d
Values obtained for calculations started with C5H5NϩCHϪOMe·H–NH3ϩ geometry.

40
Z. Dega-Szafran et al. / Journal of Molecular Structure 555 (2000) 31–42
Table 8
Calculated heat of formation (kcal), dipole moment (Debye),
˚
selected bond lengths (A), bond angles, some torsial angles (deg)
˚
and some distances (A) for N-acetyl-4-methyl-pyridinium ylide (6a)
Method
B3LYP BLYP
AM1
PM3
SAM1
a
b
DH
m
17.46
3.94
0.39
3.40
10.40
3.90
Scheme 3.
5.21
4.31
N1–C7
C7–C8
C8–O9
C2–H
1.386
1.416
1.247
1.081
1.080
1.972
2.800
2.942
123.0
115.3
114.2
130.7
0.0
1.392
1.431
1.262
1.090
1.089
1.980
2.828
2.976
122.7
115.1
113.4
132.0
0.0
1.362
1.441
1.247
1.109
1.103
2.073
2.867
2.983
123.4
116.5
116.8
125.9
0.0
1.364
1.448
1.233
1.124
1.099
1.840
2.702
2.896
1.389
1.453
1.272
1.109
1.090
1.778
2.659
2.903
121.7
116.1
114.8
132.8
0.0
where kB is the Boltzman constant and É is the Planck
constant.
C6–H
3.3. Semiempirical and DFT calculations
C2H···O9
C2···O9
N1···O9
C2–N–C7
H–C6–N1
H–C2–N1
C2–H–O9
C2–N1–C7–C8
N1–C7–C8–O9
Enthalpy, energy, dipole moment and selected
results of geometry optimisation at the PM3 and
BLYP/6-31G(d,p) level for carbocation 5, ylides 6
and their complexes with NH3, pyridine and 4-NH2-
pyridine (Scheme 2) are collected in Tables 6 and 7,
respectively. The atom numbering is given in Scheme
3. The rotational isomers shown in Fig. 3 were found
to correspond to the global minima for complexes. As
data given in Tables 6 and 7 show, there is essential
difference between the PM3 and BLYP results. The
PM3 method reproduces shorter C–O, C–H bonds
and N···O distance in comparison to the corre-
sponding BLYP values. However, geometrical differ-
ences between cations and ylides are comparable in
both methods.
121.8
118.4
115.1
129.9
0.0
0.0
0.0
0.0
0.0
0.0
a
E ˆ Ϫ440:3298074 a:u:
E ˆ Ϫ440:0491485 a:u:
b
˚
˚
bond becomes longer ca 0.04 A (BLYP) and 0.02 A
(PM3); (iii) the intramolecular Nϩ···O distance elon-
˚
˚
gates ca 0.25 A (BLYP) and 0.01 A (PM3); and (iv)
the Nϩ–C7–C8–O unit in 5 is perpendicular, while in
6 is coplanar with the pyridine ring plane. These data
indicate that in ylides, there is strong electron
coupling between the pyridine ring and the oxygen
atom through bonds, while in carbocations there is
electrostatic interaction between the positively
charged nitrogen atom and the oxygen atom via
space. Conformation of N-(carbamoyl)pyridinium
perchlorate in crystal is similar to that in 5 [60].
In ylides 6 there is a contact between the a-proton
of pyridine ring and the oxygen atom. The question
arises is if this contact can be classified as a hydrogen
The data in Tables 6 and 7 show that formation of
ylide 6 from cation 5 caused the following geometry
changes: (i) the Nϩ–C7 and C7–C8 bonds become
˚
shorter ca 0.1 and 0.15 A, respectively; (ii) the C–O
Fig. 3. The optimised BLYP/6-31 G(d,p) structures of 5a, 6a, 7ak
and 9ak.
Fig. 4. The optimised PM3 structures of 7ak and 8ak.

Z. Dega-Szafran et al. / Journal of Molecular Structure 555 (2000) 31–42
41
bond. The difference between the length of C2–H and
C6–H bonds vary with the method of calculation. The
smallest difference is predicted by the B3LYP and the
largest by the PM3 methods. The data collected in
Table 8 suggest that this interaction is not hydrogen
bond. The O9···H–C2 angle (126–133Њ) is smaller
than the analogous angle in the enol form of b-dike-
tones with intramolecular hydrogen bond (147.6Њ)
[61]. The second a-pyridine proton interacts with
NH3 and pyridines (Fig. 3, 9ak). This suggests that
the a-protons are slightly more acidic than the
methine proton.
1H NMR spectra of the investigated quaternary pyri-
dinium salts and ylides are strongly affected by substi-
tuents and solvents. Ylides have planar structures and
the a-pyridine proton has contact with the oxygen
atom and this caused its downfield shift relative to
the quaternary salts. Ammonium and pyridines
interact with the second a-pyridine proton of ylides
but not with the methine proton.
Acknowledgements
The PM3 method predicts two types of complexes,
a molecular 7 and an ion-pair 8 (Fig. 4). The former is
more stable by ca 6–10 kcal/mol. According to BLYP
results only molecular complexes are formed. This
indicates that in the gas phase ammonium is not a
strong enough base to abstract proton from 5.
However, in the case of molecular complexes several
rotamers can be formed, which differ slightly in
energy and geometry (see Table 7). Formation of
molecular complexes 7 elongates the C–H bond
This work was supported by the Polish Committee
for Scientific Research (KBN), grant 3 TO9A 094 14.
Calculations were done at the Poznan Supercom-
puting and Network Centre.
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