Noncovalent interactions between isoflurane and dimethyl ether. Spectroscopic evidence of trimer formation.

The IR spectra of isoflurane + dimethyl ether mixtures dissolved in liquid Kr are registered at T ~118-160 K. The results obtained at a wide range of relative concentrations suggest the formation of complexes stabilized by non-covalent interactions of H-bond type. Large excess of DME and low temperature favor trimer formation stabilized by interactions between two DME moieties and both CH groups of isoflurane predominantly. Estimations based on ab initio calculation of spectroscopic and thermodynamic parameters confirm the experimental findings.

FTIR cryospectroscopic and ab initio studies of desflurane-dimethyl ether H-bonded complexes.

The IR spectra of mixtures of desflurane and dimethyl ether are studied with the help of FTIR cryospectroscopy in liquefied Kr at T~118-158K. Comparative analysis of the experimental data and results of ab initio calculations show that either of the two C-H groups of desflurane is involved in heterodimer formation of comparable strengths. The blue frequency shift is found for stretching vibrations of those C-H donors which directly participate in H-bond formation. Additionally the complexes are stabilized by weaker contacts between hydrogen atoms of dimethyl ether and fluorine atoms of desflurane.

Infrared spectroscopy and ab initio study of hydrogen bonded Cl3CD·N(CH3)3 complex in the gas phase.

FTIR spectra of the gas phase Cl3CD+TMA mixture have been studied at room temperature in ∼800-4000 cm(-1) frequency domain. The formation of the H-bonded Cl3CD…TMA complex has been detected. Spectroscopic parameters of the band ascribed to the complex were evaluated. MP2 frozen core ab initio calculations have been carried out with the Pople-type 6-311++G(d,p) basis set. The equilibrium geometries and harmonic vibrational frequencies of the complex were obtained using CP-corrected gradient techniques. The ''freq=anharm'' option has been tested for Cl3CD monomer and Cl3CD…TMA complex to examine possible anharmonic effects on the vibrations localized on the proton donor. The effects of Darling-Dennison and Fermi resonances on the frequency of the stretching vibration of the CH proton donor were analyzed.

Strong and weak effects caused by non covalent interactions between chloroform and selected electron donor molecules.

The FTIR spectra of chloroform (Cl(3)CH) in mixtures with various electron donors (B = CH(3)CCH, HCCH, NCCD(3), ClCD(3) and CO) have been studied in liquefied Kr. Spectroscopic evidence of weak H-bond formation has been found. The relative stability of some complexes has been evaluated from temperature studies of integrated intensities of vibrational bands attributed to monomer and complex species. A weak red shift of the stretching vibration of chloroform involved in H-bonding with CH(3)CCH and HCCH having π-electron area was observed. However, in the case of interactions with NCCD(3) and CO, a weak blue shift was detected. In most of the cases, a noticeable increase in the integrated intensity of the CH stretching band was found. Ab initio MP2/6-311++G(2d,2p) a priori counterpoise corrected calculations have been performed for a series of Cl(3)CH and B. Stationary points at the potential energy surface were examined and the structures related to the real minima have been found. The calculations reproduce the majority of experimental results. It has been found that the commonly used correlation between the frequency shift of the CH stretching vibration of the proton donor subunit and the change in CH bond length can fail in the case of the complexes characterized by a weak frequency shift effect.

Combined effect of the DeltaPhe or DeltaAla residue and the p-nitroanilide group on a didehydropeptides conformation.

Two series of dehydropeptides of the general formulae Boc-Gly-X-Phe-p-NA, Boc-Gly-Gly-X-Phe-p-NA, Gly-X-Gly-Phe-p-NA.TFA, and Boc-Gly-X-Gly-Phe-p-NA, with X = Delta(Z)Phe and DeltaAla, were studied with NMR in DMSO and CDCl(3)-DMSO, and with CD in MeOH, MeCN, and TFE. The NMR spectra measured in DMSO suggest that peptides with the DeltaPhe residue next to Phe are folded whereas peptides with Gly between DeltaPhe and Phe are less ordered. NMR spectra of DeltaAla-containing peptides indicate that these peptides are flexible and their conformational equilibria are populated by many different conformations. The CD spectra show that conformational properties of the peptides studied are distinctly influenced by a mutual position of the dehydroamino acid residue and the p-NA group. They indicate that all dehydropeptides with the DeltaPhe residue, Boc-Gly-DeltaAla-Phe-p-NA, and Boc-Gly-Gly-DeltaAla-Phe-p-NA adopt ordered conformations in all solvents studied, presumably of the beta-turn type. The last two peptides exhibit surprising chiroptical properties. Their spectra show exciton coupling-like couplets in the region of the p-NA group absorption. This shape of CD spectra suggests a rigid, chiral conformation with a fixed disposition of the p-NA group. The CD spectra indicate that Boc-Gly-DeltaAla-Gly-Phe-p-NA and Gly-DeltaAla-Gly-Phe-p-NA.TFA are unordered, independently of the solvent.

The phase transition temperatures of a liquid crystal determined from FT-IR spectra explored by principal component analysis.

The FT-IR spectra of a thin layer of pure 4-chloro-2'-hydroxy-4'-pentyloxyazobenzene (CHPAB) were studied as a function of temperature. A detailed analysis of the intensity variations was performed by a method based on principal component analysis (PCA). It was shown that the phase transition temperatures obtained by means of PCA and those determined by differential scanning calorimetry (DSC), the most widely used technique in the field, were nearly identical. The PCA results revealed that the transition from solid to a liquid crystalline (LC) phase (smectic A) is more drastic phase transition in terms of infrared absorption changes. The nematic to isotropic phase transition is much less infrared sensitive. Very much smaller absorption changes are associated with the transition between the smectic and nematic mesophases. The pattern of the intensity changes strictly is correlated with the orientation of the CHPAB molecules towards the surface windows due to the surface-induced homeotropic alignment of LC molecules. The important role of hydrogen bonding interaction on the observed transition is disclosed.

Tautomerism of sterically hindered schiff bases. Deuterium isotope effects on 13C chemical shifts.

A series of sterically hindered o-hydroxy Schiff bases derived from o-hydroxyaceto- and benzophenones with very short intramolecular hydrogen bonds were described qualitative and quantitatively by deuterium isotope effects on (13)C chemical shift, (n)DeltaC(XD), (n)DeltaF(XD), (1)J(N,H) coupling constants, deltaNCH(3) chemical shifts and UV spectra. All the investigated compounds are found to be tautomeric. The tautomeric character is described by the signs of the deuterium isotope effects on the (13)C chemical shifts. For the 3-nitro-5-chloro derivatives at low temperature, the equilibrium is shifted almost fully toward the proton transferred form in CD(2)Cl(2). Intrinsic deuterium isotope effects on chemical shifts of these compounds as well as (1)J(N,H) coupling constants suggest that a zwitterionic resonance form is dominant for the proton transferred form. Structures, (1)H, (19)F, and (13)C chemical shifts, and deuterium isotope effects on (13)C chemical shifts are calculated by ab initio methods. The potential energy functions and the total deuterium isotope effects are calculated, and they are shown to correspond well with the experimental findings.

Liquid crystalline properties and IR spectra of 2'-hydroxy-4'-octyloxyazobenzenes.

New azobenzene derivatives, namely 3,5-dichloro, 2,6-dimethyl and 3-chloro-4-methyl derivatives of 2'-hydroxy-4'-octyloxyazobenzene were synthesized and their properties were compared with those of 4-chloro, 4-methyl and 4-nitroderivatives. Special attention was paid to the characteristics of intramolecular O-H...N hydrogen bond reflected in IR spectra of CCl(4) solutions and neat crystalline samples. The spectra were analyzed based on DFT calculations. In all cases of solutions very strong hydrogen bonds are manifested in broad bands centered at about 2700-2800 cm(-1) of low intensity typical of resonance assisted H-bonds. On the other hand, in cases of neat crystalline samples, a broad, intensified absorption is extended down to approximately 600 cm(-1) forming a continuum resembling a Hadzi's trio. However, neither broad maxima nor minima can be assigned to delta(OH) or gamma(OH) overtones. The studies of the H/D isotope effect on the continuum seem to indicate on the role of resonance couplings between nu(OH) vibrators of neighboring molecules as well as hot bands arising from the coupling between high and low frequency modes. These couplings modify the dynamic pattern of the potential for the proton motion leading to a decrease of the barrier for the proton transfer.

Near infrared spectra (4000-10 500 cm-1) of phenol-OH and phenol-OD in carbon tetrachloride.

The near infrared spectra (3800-10 500 cm(-1) of phenol-OH and phenol-OD are studied in carbon tetrachloride solution. The bandwidth of the v(OH) and v(OD) stretching vibrations increases with the vibrational quantum number in contrast to results obtained by nonresonant ionization spectroscopy (S.I. Ishiuchi et al., Chem. Phys. Lett. 283 (1998) 243). The bandwidth of the v(CH) vibrations obtained by a deconvolution procedure also increases with the frequencies associated with the vibrational transitions. The anharmonicity of the v(CH) vibrations ranges between 51 and 72 cm(-1). Numerous absorptions are observed in the near infrared spectra. These absorptions are tentatively assigned to combinations involving the fundamental transitions which have been recently calculated at different levels of theory (D. Michalska et al., J. Phys. Chem. 100 (1996) 17786). The experimental, theoretical and harmonic v(OH) and vi(CH) frequencies are compared.

Conformational investigation of alpha,beta-dehydropeptides. VIII. N-acetyl-alpha,beta-dehydroamino acid N'-methylamides: conformation and electron density perturbation from infrared and theoretical studies.

The Fourier transform infrared spectra are analyzed in the regions of Vs(N-H), amide I, amide II and Vs(C alpha = C beta) bands for a series of Ac-delta Xaa-NHMe, where delta Xaa = delta Ala, (Z)-delta Abu, (Z)-delta Leu, (Z)-delta Phe and delta Val, to determine the predominant solution conformation of these alpha,beta-dehydropeptide-related molecules and the electron distribution perturbation in their amide bonds. The measurements were performed in dichloromethane (DCM). To confirm and rationalize the assignments, the spectra of the respective series of saturated Ac-Xaa-NHMe, recorded in DCM, and the spectra of these two series of unsaturated and saturated compounds, recorded in acetonitrile, were examined. To help interpret the spectroscopic results, the equilibrium geometrical parameters for some selected amides were used. These were optimized with ab initio methods in the 6-31G** basis set. Each of the dehydroamides studied adopted a C5 structure, which in Ac-delta Ala-NHMe is fully extended and accompanied by the strong C5 hydrogen bond. Interaction with the C alpha = C beta bond lessened the amidic resonance within each of the flanking amide groups. The N-terminal C = O bond was noticeably shorter, both amide bonds were longer than the corresponding bonds in the saturated entities and the N-terminal amide system was distorted. Ac-delta Ala-NHMe constituted an exception. Its C-terminal amide bond was shorter than the standard one and both amide systems were prototypically planar.

Assignment of the Vibrations in the Near-Infrared Spectra of Phenol-OH(OD) Derivatives and Application to the Phenol-Pyrazine Complex

The near-infrared spectra (7200-3800 cm-1 ) of seven phenols OH-OD (4-CH3 , H, 4-Cl, 4-Br, 3-Br, 3,4-diCl, 3-5-diCl) are investigated at room temperature in carbon tetrachloride. The mid-infrared spectra of some phenol-OD have also been studied. Beside the first overtone of the nu(OH) stretching vibration, six absorptions disappear upon deuteration. The absorptions are assigned to combinations involving the nu(OH) stretching mode and aromatic ring vibrations, the nu(C-O) mode, or the delta(OH) mode. The anharmonicity and coupling constants are discussed. The isotopic ratio of the nu(OH) stretching fundamentals, of their first overtone, and the ratio of the anharmonicity constants are very insensitive to the nature of the phenol. The stability constants of complexes between phenols and base can be computed from several near-infrared absorptions and this is illustrated for the 3,5-dichlorophenol-pyrazine complex.

Changes of dipole moments in the series of Boc-(ProLeuGly)n-OBzl oligopeptides and their possible explanation. A preliminary survey.

It was found that the dipole moments in the series of Boc-(ProLeuGly)n-OBzl oligopeptides (n = 1-5) measured in dioxane solution demonstrate a distinct concentration dependence beginning from dodecapeptide. This effect may be connected with the appearance of aggregated (triple-helical?) structure in conformation equilibrium. The tendency to minimize the dipole moment of the system may be of some importance in the creation of supercoiled structures of polypeptides.