Amines vibrational frequencies

The IEs on amine basicity are also due to changes in vibrational frequencies, not only computationally but also experimentally. Gas-phase IEs of O.lOkcalmol-1 per CD3 group on basicities of methylamine, dimethy-lamine, and trimethylamine can be reproduced by ab initio force constants for C-H stretching, which increase on TV-protonation.100 Infrared spectra of amines show characteristic bands (called Bohlmann bands ) in the 2700-2800 cm-1 region, lower than the 2900 cm-1 of a typical C-H stretch.169,170 Upon /V-protonation these bands revert to a typical, higher frequency. Therefore the zero-point energy of the C-H increases on... 

The weakening of a bonds by negative hyperconjugation with lone electron pairs also reveals itself in IR and NMR spectra. Thus, C-H, N-H, or O-H bonds oriented trans or antiperiplanar to an unshared, vicinal electron pair are weakened and have therefore a significantly reduced IR vibrational frequency [17]. The C-H vibrational frequency in aldehydes is, for example, lower than that in alkenes (Scheme 2.7). Polycyclic amines with at least two hydrogen atoms antiperiplanar to the lone pair on nitrogen have characteristic absorption bands at 2800-2700 cm-1 which have been used to infer the relative configuration of such amines [18]. 

The 0 — H bonds of alcohols and the N—H bonds of amines are strong and stiff. The vibration frequencies of O—H and N—H bonds therefore occur at higher frequencies than those of most C—H bonds (except for alkynyl =C—H bonds). 

Larson and Cremer have explored another approach to dissecting BDE into inherent and RSE effects4 There is a relationship between C-H BDE and the vibrational frequencies of the bonds Furthermore, the vibrations can be determined for C—H bonds in speeific conformations, for example, the equatorial and axial bonds in cyclohexanes or the anti and gauche bonds in amines. 

The N—H stretching vibration corresponds to an infrared band between 3500 and 3400 cm in the spectra of secondary aliphatic amines taken in dilute solution in carbon tetrachloride to avoid molecular association . Under these conditions primary aliphatic amines show two bands near 3500 and 3400 cm (asymmetrical and symmetrical vibrations) Among the vibration frequencies associated with ammonia two harmonic frequencies are found in approximately the same region (3506 and 3577 cm ) Therefore we can anticipate that the main force constant RR will be of the same order of magnitude for NHg, as well as for secondary and primary amines. This fact supports the hypothesis that the electronic structure of the N—bonds would not be very different in all these molecules. 

Nucleobases, such as cytosine, thymine, and uracil are the most important pyrimidine derivatives. In a recent gas-phase study [64], three low-energy tautomers were identified on the basis of CCSD(T)/cc-pVQZ energy calculations andMP2/6-311++G(2d,2p) estimation ofthe vibrational frequencies. The authors emphasize the importance of the high-level estimation of the frequencies and the related thermal correction in order to obtain rehable AG within a 0.7 kcal mol free energy range of the tautomers at T = 450 K. The lowest-free-energy structure is the canonical hydroxy-amine form of cytosine. 

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