N—H Stretching Modes

Simple primary amides in dilute solutions display two bands due to free N—H stretching near 3500 and 3400 cm . In the solid state two bands due to bonded N—H lie near 3350 and 3180 cm (Randall et al., 1949 Clarke et al., 1949 Richards and Thompson, 1947 Darmon and Sutherland, 1949 Badger and Rubalcava, 1954). A detailed study (Cleverley, 1956) of free and associated N—H band positions in fourteen amides in chloroform solution showed that neither the position nor the intensity of the free N—H band is influenced by the length or nature of the alkyl chain. However, N—H stretching vibrations are concentration dependent. For example, n-valeramide 

Secondary amides have a free N—H stretching band in the range 3470-3400 cm in dilute solutions. The frequency of bonded NH absorptions depends on the nature of the solvent and upon the concentration. With increasing concentration, two bonded NH bands are found at 3340-3140 and 3100-3060cm (Randall et a/., 1949 Clarke et al, 1949 Richards and Thompson, 1947 Darmon and Sutherland, 1949). The spectra of polypeptides and proteins also have these bands. Two bands that appear as a doublet in secondary amides are due to cis- and trans-rotational isomers containing the free N—H stretching band (Russell and Thompson, 1956). 

N-Methylacetamide and other simple secondary amides are found predominantly in the trans configuration. N-f-Butylphenylacetamide and other sterically hindered amides are found predominantly in the cis form. In open-chain secondary amides, the principal N—H stretching band lies near 3270cm in the solid state (Richards and Thompson, 1947 Darmon and Sutherland, 1949 Letaw and Gropp, 1953). 

All primary amides display the amide I band, usually between 1715 and 1675 cm in solution and around 1650cm in the solid state. For n-alkyl amides CH3(CH2) C0NH2 (where n = 1 to 10) the band is observed at 1679 cm in dilute 

Open-chain secondary N-monosubstituted amides display the C=0 stretching band from 1680 to 1630 cm Mn the solid state, and from 1700 to 1650 cm Mn dilute solution. This band has been studied extensively, but the correlations are mainly applicable to solids (Randall et al, 1949 Thompson et al, in Clarke et al, 1949). The groups substituted on the nitrogen produce inductive and conjugation effects which influence the C=0 frequency. For example, N-chloroacetamide has its carbonyl band at 1705cm (Gierer, 1953), while N-aryl substituted amides absorb near 1700cm (Richards and Thompson, 1947). Jones and Sandorfy (1956) have discussed the amide I band in N,N-disubstituted amides (tertiary amides, see Table 

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In a symmetric top molecule such as NH3, if the transition dipole lies along the molecule s symmetry axis, only k = 0 contributes. Such vibrations preserve the molecule s symmetry relative to this symmetry axis (e.g. the totally symmetric N-H stretching mode in NH3). The additional selection rule AK = 0... 

Infrared The absorptions of interest m the IR spectra of amines are those associated with N—H vibrations Primary alkyl and arylammes exhibit two peaks m the range 3000-3500 cm which are due to symmetric and antisymmetric N—H stretching modes... 

Zengin et al. [41] characterized a polyaniline (PANI)/MWNT composite. The FTIR spectra of the composite film show benzoid and quinoid ring vibrations at 1500cm-1 and 1600 cm-1, respectively, which indicate the presence of emeraldine salt (ES) of polyaniline. A weak broad band near 3400 cm-1 is assigned to the N—H stretching mode. The strong band at 1150cm-1 is characteristic of PANI conductivity. The FTIR spectrum of PANI/MWNT composite in the ES form exhibits several clear differences from the spectrum of neat ES PANI (1) the composite spectrum shows an inverse... 

Note Another N-H stretching mode //(Ne-Hg) weakly coupled to z/(N6-H6) becomes red-shifted by only 55 cm"... 

M. Shmilovits-Ofir, R. B. Gerber. Proton transfer and dissociation of GlyLysH(- -) following O-H and N-H stretching mode excitations dynamics simulations, J. Am. Chem, Soc., 133 ... 

A third unexpected feature is less readily rationalized. In gaseous ammonia the absorption coefficient of the symmetric N—H stretching mode exceeds the absorption coefficient of the asymmetric stretching mode. It has been noted that in the spectrum of solid ammonia these intensities are reversed The asymmetric mode being stronger [6] In the matrix spectra this intensity reversal is also observed, even for the monomeric species. 

Early studies of deuteriation (and protiodedeuteriation), used infrared (IR) (which is less convenient) to measure changes in the intensity of the C—H stretching frequencies. This technique was used to study the kinetics of N—H exchange in azoles [75JCS(P2)1316], the decrease in the first overtone band of the N—H stretching mode at 1.48 p-m being followed. 

Next we will see what kind of internal coordinate changes can account for each of these normal modes. There will have to be two N-H stretching modes and one N-N stretching mode. For deformation... 

Two papers have attempted to compare the observed infrared lines of the base pairs in the region of the hydrogen-bond absorption with the calculated data. Rein and Svetina have calculated the proton vibrational states and relative transition probabilities for two guanine-cytosine hydrogen bonds. Their preliminary results seemed to be consistent with the absorption peak at 3489 cm (0.436 eV) reported by Pitha et for the hydrogen bond N-H stretching mode of... 

They were, for example, very much more intense that the H-N-H bending modes and the N-H stretching modes For this reason, my assignments were disputed at a number of conferences. Sometime later, I argued that this intensity arose because the hydrogen motion moved the nitrogen lone pair out of the metal-nitrogen axis [16], this was later confirmed by a crude but instructive calculation [17],... 

Mexiletine hydrochloride has been found to crystallize in six polymorphic forms, each of which yields a characteristic IR spectrum [118], As shown in the data collected in Table 4, the very intense bands due to the C-N(H) stretching mode (1020-1060 cm ) and the aromatic C-H out-of-plane deformation mode (760-780 cm ) were found to be particularly sensitive to the structural differences. In fact, the splitting of the latter mode observed in four of the six polymorphs suggests the existence of different types of molecules within the unit cell. The data in Table 4 also illustrate the typical magnitudes in band energy differences encountered within the vibrational spectra of polymorphic systems. 

Investigated systems include a broad variety of dissolved organometallic species, electrochemically active organic molecules and redox active polymers like polyani-line (for a review, see [142]). Both dissolved species and species attached by adsorption, covalent bonding or film-forming deposition have been studied. Dissolved poly aniline dispersions as prepared by chemical oxidation [145] show various transitions in the NIR, as depicted in a set of NIR spectra in Fig. 5.40. The bands around X = 1490 nm and 1950 nm are overtones of the N-H stretch mode of an aromatic amine, whereas the band around X = 2300 nm is caused by the oligomer itself, which presumably indicates the presence of mobile charge carriers. 

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