Modern Polymer Spect..

5.3 Ainide Modes 251
Table 5-1. Amide modes of isolated and aqueous hydrogen-bonded N-Methylacetamide"
Mode v(obsib v(calc)c Potential energy distributiond
I 1706
1646
1626
I1 1511
1584
111 1265
1313
IV 658
632
V 391
(7501"
v1 626
(600)'
VII ~
195g
1708
1632
1626
1512
1575
1264
1305
648
640
391
744
627
605
-
20s
CO ~(83) CCN d(l1)
CO s(30j [O)HOH b(30) fH)HOH b(18)
CO s(38) (0)HOH b(28) (H)HOH b(1S)
NH ib(5l) CN s(28)
CN s(48j NH ib(36)
NH lb(21) CO lb(21) CN ~(18) CC S(10) CCH3 sb(l0)
NH lb(26) CN ~(20) CCHi sb(20) CO lb(12)
CC ~(36) CO ib(34)
CO ib(35) CC s(32)
CN t(118) NH ob(51) CO ob(23)
CN t(43) H 0 b2128) NH ob(21)
CO ob(67) CCH3 or(26) CN t( 11)
CO ob(50) 0 . H b2(30) CCH3 OI(19) NH ob(l0)
CN t(38) NH ob(21)
First line: NMA; second line(s): NMA-(H20)2. 6-31+G* basis set.
NMA: from [59]; NMA-(H20)2: from 1611 and 1681, except where noted.
NMA: from [73]: NMA-(H20)2: from [64].
s: stretch, d: deformation. b: bend. ib: in-plane bend, sb: synmetric bend, ob: out-of-plane bend,
b2: essentially ob, or: out-of-plane rock, t: torsion. Group coordinates are defined in [59]. Contributions
> 10.
Possible value, from 1691.
Observed in neat NMA [65].
From 1621 (at 206 cm-' from [70]1.
(FA)2 this contribution is NH in-plane bend (ib) [63], but such a result is uncertain
because of the absence of experimental data on this complex and the resulting lack
of optimization of the FA force constants. Nevertheless, we see that even for as
localized a mode as CO s the specific nature of the eigenvector can depend on the
details of the calculation.
The amide I1 mode is a mixture of NH ib and CN s, with the former dominating
in NMA and the latter, at a higher hydrogen-bonded frequency, dominating in
NMA-(HzO)?, (Table 5-1). It is interesting that the CN s predominance does
not appear at 3-21G for NMA-(FA)> [63], nor at 4-31G [62] or 4-31G" [61] for
NMA-(H20)2, showing the evident influence of basis set on the predicted eigenvectors.
Nor is CN s predominant in polypeptides [5].
The amide I11 mode is also a mixture of NH ib and CN s, although other coordinates
make significant contributions. Since CH3 symmetric bend (sb) is an important
one, CH3 point-mass calculations [52, 51 distort the relative importance of
CC s. As seen from Table 5-1, CO ib is a relatively large contributor in NMA and a