High level ab initio quantum chemical calculations have been performed on the association of chloroform with ammonia in the gas phase (counterpoise corrected MP2 and coupled-cluster CCSD(T) calculations with 6-311++G(d,p) basis functions). Minimum energy equilibrium structures have been found for CHCl(3)-NH(3) dimer, CHCl(3)-(NH(3))(2) trimer and CHCl(3)-(NH(3))(3) tetramer. Association is characterised by a CHN hydrogen bond between a chloroform and an ammonia molecule, with further ammonia units attached by hydrogen bonds to ammonia and by an electrostatic NHCl interaction to chloroform. Cooperative effects provide additional stabilisation. The complexes exhibit characteristic shifts of vibrational bands and change of IR intensity; in particular there is a pronounced progressive shift of the CH-stretching vibration towards lower wavenumber with each unit of ammonia attached in the complex. The shift is accompanied by an up to 600 fold increase in IR intensity. The experimental FTIR jet spectra have provided firm evidence of CHCl(3)-NH(3) association, with the clearest effects seen in the region of the CH-stretching vibration. First tentative assignments have been made based on the dependence of relative intensities of cluster features on the concentration of monomers, and assignments have been corroborated by the quantum chemical calculations. The present combined ab initio and FTIR spectroscopy study reveals the structure and energetics of cluster formation and intermolecular bonding in CHCl(3)-NH(3) association.

The formation of C-H...π bonded complexes of halothane with benzene(-d(6)) has been studied using infrared and Raman spectroscopy of solutions in liquid krypton, in supersonic jet expansions and in room temperature vapour phase. The formation of complexes with 1 ∶ 1 and 2 ∶ 1 stoichiometry was observed. The complexation enthalpy in liquid krypton for the 1 ∶ 1 complex was determined to be -9.8(2) kJ mol(-1) and the enthalpy for the addition of a second halothane molecule to the 1 ∶ 1 complex was determined at -7.0(3) kJ mol(-1). The stretching mode of the halothane C-H bond involved in the formation of the complex in the jets was observed to blue shift by 7.7(10) cm(-1). In contrast, for the solutions of liquid krypton and the room temperature measurements a small red shift was observed. Supported by ab initio calculations and Monte Carlo simulations, this shift was explained by the differences in thermal populations of the van der Waals vibrations of the complex in the different experiments.

The dramatically lower volatility of gamma-butyrolactone compared to its open chain analog methyl propionate is analyzed at the molecular dimer level using FTIR spectroscopy in supersonic jets. It is found that the spectral shifts from the monomer to the dimer are about three times more pronounced in the lactone at low temperatures. The spectra are consistent with sandwich-like dimers optimizing their strong dipole-dipole interaction, possibly augmented by specific C-H...O=C hydrogen bond contacts. The spectra show significant evolution from the dimer to the condensed phase, indicative of secondary interactions with the ester oxygen and long range forces. The reduced dipole moment in the open chain ester leads to less specific interactions, unless a trans conformation of the ester group as in the lactones is enforced. The latter is not energetically accessible in open chain esters because it would bring the molecular C=O and C-O-C dipole moments into an unfavorable near-parallel orientation, thus their higher volatility.

Aggregation in hydroxyacetone (HA) is studied using low-temperature FTIR, supersonic jet expansion, and X-ray crystallographic (in situ cryocrystallization) techniques. Along with quantum chemical methods (MP2 and DFT), the experiments unravel the conformational preferences of HA upon aggregation to dimers and oligomers. The O-H···O═C intramolecular hydrogen bond present in the gas-phase monomer partially opens upon aggregation in supersonic expansions, giving rise to intermolecular cooperatively enhanced O-H···O-H hydrogen bonds in competition with isolated O-H···O═C hydrogen bonds. On the other hand, low-temperature IR studies on the neat solid and X-ray crystallographic data reveal that HA undergoes profound conformational changes upon crystallization, with the HOCC dihedral angle changing from ~0° in the gas phase to ~180° in the crystalline phase, hence giving rise to a completely new conformation. These conclusions are supported by theoretical calculations performed on the geometry derived from the crystalline phase.

Esters of glycine, alanine and valine are investigated by FTIR and Raman spectroscopy in supersonic jets as gas phase model systems for the neutral peptide N-terminus. The NH-stretching vibrations exhibit very large temperature- and substitution-dependent intensity anomalies which are related to weak, bifurcated intramolecular hydrogen bonds to the carbonyl group. Comparison to theory is only satisfactory at low temperature. Spectral NH aggregation shifts are small or even negligible and the associated IR intensity is remarkably low. In the case of valine, chirality recognition effects are nevertheless detected and rationalized. Comparison to quantum-chemical calculations for dimers shows that dispersion interactions are essential. It also rules out cooperative hydrogen bond topologies and points at deficiencies in standard harmonic treatments with the linear dipole approximation.

A vibrational assignment of the anaesthetic sevoflurane, (CF(3))(2)CHOCH(2)F, is proposed and its interaction with the aromatic model compound benzene is studied using vibrational spectroscopy of supersonic jet expansions and of cryosolutions in liquid xenon. Ab initio calculations, at the MP2/cc-pVDZ and MP2/aug-cc-pVDZ levels, predict two isomers for the 1 : 1 complex, one in which the near-cis, gauche conformer of sevoflurane is hydrogen bonded through its isopropyl-hydrogen atom, the other in which the same conformer is bonded through a bifurcated hydrogen bond with the fluoromethyl hydrogen atoms. From the experiments it is shown that the two isomers are formed, however with a strong population dominance of the isopropyl-bonded species, both in the jet and liquid phase spectra. The experimental complexation enthalpy in liquid xenon, ΔH(o)(LXe), of this species equals -10.9(2) kJ mol(-1), as derived from the temperature dependent behaviour of the cryosolution spectra. Theoretical complexation enthalpies in liquid xenon were obtained by combining the complete basis set extrapolated complexation energies at the MP2/aug-cc-pVXZ (X = D,T) level with corrections derived from statistical thermodynamics and Monte Carlo Free Energy Perturbation calculations, resulting in a complexation enthalpy of -11.2(3) kJ mol(-1) for the isopropyl-bonded complex, in very good agreement with the experimental value, and of -11.4(4) kJ mol(-1), for the fluoromethyl-bonded complex. The Monte Carlo calculations show that the solvation entropy of the isopropyl-bonded species is considerably higher than that of the fluoromethyl-bonded complex, which assists in explaining its dominance in the liquid phase spectra.

The formation of C-H···N bonded complexes of halothane with ammonia has been studied using infrared and Raman spectroscopy of solutions in the liquid rare gases argon, krypton and xenon, of supersonic jet expansions and of room temperature vapor phase mixtures. For the solutions and for the vapor phase experiments, the formation of complexes with 1:1 and 1:2 stoichiometry was observed. The complexation enthalpy for the 1:1 complex was determined to be -20 (1) kJ mol(-1) in the vapor phase, -17.0 (5) kJ mol(-1) in liquid xenon and -17.3 (6) kJ mol(-1) in liquid krypton. For the 1:2 complex in liquid xenon, the complexation enthalpy was determined to be -31.5 (12) kJ mol(-1). Using the complexation enthalpies for the vapor phase and for the solutions in liquid xenon and krypton, a critical assessment is made of the Monte Carlo Free Energy Perturbation approach to model solvent influences on the thermodynamical properties of the cryosolutions. The influences of temperature and solvent on the complexation shifts of the halothane C-H stretching mode are discussed.

The N-H stretching signature of the puckering equilibrium between equatorial and axial pyrrolidine is analyzed via FTIR and Raman spectroscopy in supersonic jets as a function of aggregation. Vibrational temperatures along the expansion axis can be extracted from the Raman spectra and allow for a localization of the compression shock waves. While the equatorial conformation is more stable in the ground state monomer, this preference is probably switched in the excited state with one N-H stretching quantum. Furthermore, the dominant dimer involves an axial donor and the trimer and tetramer structures seem to prefer uniform axial conformations. The IR intensity is boosted by up to 3 orders of magnitude upon aggregation, whereas the Raman scattering intensity shows only moderate hydrogen bond effects. B3LYP and MP2 calculations provide a reasonable description of the N-H vibrational dynamics under the influence of self-aggregation. In mixed dimers with pyrrole, pyrrolidine assumes the role of a hydrogen bond acceptor.