H/D isotopic recognition mechanism in hydrogen-bonded crystals of 3-methylacetanilide and 4-methylacetanilide.

Polarized IR spectra of 3- and 4-methylacetanilide as well as their deuterium derivative crystals were measured at 293K and at 77K by a transmission method. The obtained results were interpreted within the limits of the "strong-coupling" theory. This approach facilitated the understanding of the H/D isotopic, temperature and dichroic effects observed in the hydrogen bond IR spectra. The existence of H/D isotopic "self-organization" phenomenon, depending on the non-random distribution of protons and deuterons in the crystal lattices of isotopically diluted samples of a compound was ascertained. This effect resulted from the dynamical co-operative interactions involving the closely spaced hydrogen bonds, each belonging to a different chain of associated 3- and 4-methylacetanilide molecules. In the case of 4-methylacetanilide crystals weaker but non-negligible exciton coupling also involved adjacent hydrogen bonds in each molecular chain and the H/D isotopic "self-organization" mechanism concerned at least four hydrogen bonds from each unit cell. The source of these phenomena was ascribed to the molecular electronic properties determined by aromatic rings linked to nitrogen atoms of the amide fragments.

Effects of dynamical couplings in IR spectra of the hydrogen bond in N-phenylacrylamide crystals.

This article presents the investigation results of the polarized IR spectra of the hydrogen bond in N-phenylacrylamide crystals measured in the frequency range of the proton and deuteron, ν(N-H) and ν(N-D), stretching vibration bands. The basic spectral properties of the crystals were interpreted quantitatively in terms of the "strong-coupling" theory. The proposed model of the centrosymmetric dimer of hydrogen bonds facilitated the explanation of the well-developed, two-branch structure of the ν(N-H) and ν(N-D) bands as well as the isotopic dilution effects in the spectra. The vibronic mechanism of the generation of the long-wave branch of the ν(N-H) band ascribed to the excitation of the totally symmetric proton vibration was elucidated. The complex fine structure pattern of ν(N-H) and ν(N-D) bands in N-phenylacrylamide spectra in comparison with the spectra of other secondary amide crystals (e.g., N-methylacetamide and acetanilide) can be accounted for in terms of the vibronic model for the forbidden transition breaking in the dimers. On the basis of the linear dichroic and temperature effects in the polarized IR spectra of N-phenylacrylamide crystals, the H/D isotopic "self-organization" effects were revealed.

Investigations of interhydrogen bond dynamical coupling effects in the polarized IR spectra of acetanilide crystals.

This Article presents the investigation results of the polarized IR spectra of the hydrogen bond in acetanilide (ACN) crystals measured in the frequency range of the proton and deuteron stretching vibration bands, nu(N-H) and nu(N-D). The basic spectral properties of the crystals were interpreted quantitatively in terms of the "strong-coupling" theory. The model of the centrosymmetric dimer of hydrogen bonds postulated by us facilitated the explanation of the well-developed, two-branch structure of the nu(N-H) and nu(N-D) bands as well as the isotopic dilution effects in the spectra. On the basis of the linear dichroic and temperature effects in the polarized IR spectra of ACN crystals, the H/D isotopic "self-organization" effects were revealed. A nonrandom distribution of hydrogen isotope atoms (H or D) in the lattice was deduced from the spectra of isotopically diluted ACN crystals. It was also determined that identical hydrogen isotope atoms occupy both hydrogen bonds in the dimeric systems, where each hydrogen bond belongs to a different chain. A more complex fine structure pattern of nu(N-H) and nu(N-D) bands in ACN spectra in comparison with the spectra of other secondary amides (e.g., N-methylacetamide) can be explained in terms of the "relaxation" theory of the IR spectra of hydrogen-bonded systems.

Theoretical modeling of infrared spectra of the hydrogen and deuterium bond in aspirin crystal.

An extended quantum theoretical approach of the nu(X-H) IR lineshape of cyclic dimers of weakly H-bonded species is proposed. We have extended a previous approach [M.E.-A. Benmalti, P. Blaise, H.T. Flakus, O. Henri-Rousseau, Chem. Phys. 320 (2006) 267] by accounting for the anharmonicity of the slow mode which is described by a "Morse" potential in order to reproduce the polarized infrared spectra of the hydrogen and deuterium bond in acetylsalicylic acid (aspirin) crystals. From comparison of polarized IR spectra of isotopically neat and isotopically diluted aspirin crystals it resulted that centrosymmetric aspirin dimer was the bearer of the crystal main spectral properties. In this approach, the adiabatic approximation is performed for each separate H-bond bridge of the dimer and a strong non-adiabatic correction is introduced into the model via the resonant exchange between the fast mode excited states of the two moieties. Within the strong anharmonic coupling theory, according to which the X-H...Y high-frequency mode is anharmonically coupled to the H-bond bridge, this model incorporated the Davydov coupling between the excited states of the two moieties, the quantum direct and indirect dampings and the anharmonicity for the H-bond bridge. The spectral density is obtained within the linear response theory by Fourier transform of the damped autocorrelation functions. The evaluated spectra are in fairly good agreement with the experimental ones by using a minimum number of independent parameters. The effect of deuteration has been well reproduced by reducing simply the angular frequency of the fast mode and the anharmonic coupling parameter.

Quinolin-6-ol at 100 K.

The title compound, C(9)H(7)NO, has two symmetry-independent molecules in the asymmetric unit, which have different conformations of the hydroxy group with respect to the quinoline ring. One of the molecules adopts a cis conformation, while the other shows a trans conformation. Each type of independent molecule links into a separate infinite O-H...N hydrogen-bonded chain with the graph-set notation C(7). These chains are perpendicular in the unit cell, one extended in the a-axis direction and the other in the b-axis direction. There is also a weak C-H...O hydrogen bond with graph-set notation D(2), which runs in the c-axis direction and joins the two separate O-H...N chains. The significance of this study lies in the comparison drawn between the experimental and calculated data of the crystal structure of the title compound and the data of several other derivatives possessing the hydroxy group or the quinoline ring. The correlation between the IR spectrum of this compound and the hydrogen-bond energy is also discussed.

N-Benzylformamide at 150 K.

The title compound, C(8)H(9)NO, crystallizes with Z' = 2. Each type of independent molecule links into a separate N-H...O hydrogen-bonded chain in the a-axis direction. There are also three weak C-H...O hydrogen bonds, which join the molecules into a two-dimensional sheet parallel to (001). The molecules exhibit the trans conformation of the -NHCHO group and an anti conformation around the (Ph)C-NH(CHO) bond. The formamide group in each of the symmetry-independent molecules is twisted out of the benzyl group plane, forming angles of 75.96 (10) and 65.23 (11) degrees with this plane. The significance of this study lies in the comparison drawn between the experimental and calculated data of the crystal structure of the title compound and the data of several other derivatives possessing the -CH(2)-NH-CO- group. The correlation between the IR spectrum of this compound and the hydrogen-bond energy is also discussed. This molecular system is of particular interest to biochemists because of its preventative function against toxic products of alcohols in human metabolism.

Thioacetanilide at 120 K.

The title compound, C(8)H(9)NS, has four symmetry-independent molecules in the asymmetric unit. These molecules link into two independent infinite N-H...S hydrogen-bonded chains in the a-axis direction with graph-set notation C(2)(2)(8). The NH-CS group adopts a trans conformation and forms a dihedral angle of about 50 degrees with the phenyl ring. The intermolecular hydrogen-bond energy calculated by the density functional theory (DFT) method is -14.95 kJ mol(-1). The correlation between the IR spectrum of this compound and the hydrogen-bond energy is also discussed. This molecular system is of interest because of its biological function.