Yl. Wang et Ra. Poirier, FACTORS THAT INFLUENCE THE C=N STRETCHING FREQUENCY IN IMINES, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(5), 1997, pp. 907-912
The factors that influence the C=N stretching frequency and the C=N pi
-bond strength of unprotonated and protonated imines have been studied
using generalized valence bond (GVB) calculations. For simple imines,
such as, CH2=NH and trans-CH3CH=NCH3, the C=N stretching force consta
nt increases upon protonation, whereas for polyimines (CH2=(CHCH=)(n)N
H), the C=N stretching force constant decreases by 0.47 mdyn/Angstrom
upon protonation. The calculations clearly show that the C=N(H) stretc
hing frequency and its deuterium isotope shift in protonated imines ar
e significantly influenced by the C=NH bending and the C=N/C=NH coupli
ng force constants. The increase in the C=N(H) stretching frequency up
on protonation is shown to be neither due to a negative C=N/C=NH coupl
ing force constant nor due to the C=N stretching but to be mainly due
to the strong contribution from the large C=NH bending force constant
(1.08 mdyn/rad(2)) and a positive coupling constant (0.30 mdyn/rad). T
his C=NH bending contribution also affects the C=N(H) stretching frequ
ency deuterium isotope shift. The GVB calculations predict a deuterium
isotope shift of 26 cm(-1) in the C=N(H) stretching frequency and a N
-15 isotope shift of 18 cm(-1), which are in excellent agreement with
the corresponding experimental values of 25 and 15 cm(-1), respectivel
y.