The harmonic approximation provides a powerful approach for interpreting vibrational spectra. In this treatment, the energy and intensity of the 3N- 6 normal modes are calculated using a quadratic expansion of the potential energy and a linear expansion of the dipole moment surfaces, respectively. In reality, transitions are often observed that are not accounted for by this approach (e.g. combination bands, overtones, etc.), and these transitions arise from inherent anharmonicities present in the system. One interesting example occurs in the vibrational spectrum of H(2)O((l)), where a band is observed near 2000 cm(-1) that is commonly referred to as the "association band". This band lies far from the expected bend and stretching modes of the water molecule, and is not recovered at the harmonic level. In a recent study, we identified a band in this spectral region in gas-phase clusters involving atomic and molecular adducts to the H(3)O(+) ion. In the current study we probe the origins of this band through a systematic analysis of the argon-predissociation spectra of H(3)O(+)·X(3) where X = Ar, CH(4), N(2) or H(2)O, with particular attention to the contributions from the non-linearities in the dipole surfaces, often referred to as non-Condon effects. The spectra of the H(3)O(+) clusters all display strong transitions between 1900-2100 cm(-1), and theoretical modeling indicates that they can be assigned to a combination band involving the HOH bend and frustrated rotation of H(3)O(+) in the solvent cage. This transition derives its oscillator strength entirely from strong non-Condon effects, and we discuss its possible relationship to the association band in the spectrum of liquid water.

2010) Vibrational predissociation spectra of the Ar-tagged [CH 4 · H 3 O + ] binary complex: spectroscopic signature of hydrogen bonding to an alkane, Vibrational predissociation spectra of the Ar-tagged [H 3 O þ Á X], X ¼ CH 4 , CD 4 , N 2 , and Ar complexes are analysed to explore the hydrogen bonding acceptor properties of an alkane. The observed red shift in the OH stretching transition of the donor is found to be significantly smaller than anticipated by the previously reported trend in this value with the proton affinity of the acceptor [Science 316, 249 (2007)] [1]. Specifically, the alkane-induced red shift of the OH stretching frequency is less than that caused by the conventional proton acceptor, N 2 , even though the latter is a weaker base than methane. The origin of this effect is discussed in the context of the structures of the complexes and the molecular rearrangements required for complete proton transfer to hydrocarbons as opposed to the situation in conventional H-bond acceptors.

The structural, vibrational, and energetic properties of adducts of alkanes and strong cationic proton donors were studied with composite ab initio calculations. Hydrogen bonding in D-H(+)...H-alkyl adducts contributes to a significant degree to the interactions between the two components, which is substantiated by NBO and AIM results. The hydrogen bonds manifest themselves in the same manner as conventional hydrogen bonds, D-H bond elongation, D-H vibrational stretching frequency red shift and intensity increase, and adduct stabilization. The alkane adducts also exhibit elongation of the C-H bonds involved and a concurrent red shift, which is rationalized in terms of charge-transfer interactions that cause simultaneous weakening of both the O-H and C-H bonds. Like other dihydrogen-bonded adducts, the adducts possess a bent structure and asymmetric bifurcated hydrogen bonds. The hydrogen bonds are stronger in adducts of isobutane and in adducts of stronger acids. Intramolecular hydrogen bonding in protonated long-chain alcohols manifests itself in the same manner as intermolecular hydrogen bonding and can be equally strong.

It is generally expected that the hydrogen bond strength in a D-H(***)A adduct is predicted by the difference between the proton affinities (DeltaPA) of D and A, measured by the adduct stabilization and demonstrated by the infrared (IR) redshift of the D-H bond stretching vibrational frequency. These criteria do not always yield consistent predictions, as illustrated by the hydrogen bonds formed by the E and Z OH groups of protonated carboxylic acids. The DeltaPA and the stabilization of a series of hydrogen bonded adducts indicate that the E OH group forms the stronger hydrogen bonds, whereas the bond length changes and the redshift favor the Z OH group, matching the results of NBO and AIM calculations. This reflects that the thermochemistry of adduct formation is not a good measure of the hydrogen bond strength in charged adducts, and that the ionic interactions in the E and Z adducts of protonated carboxylic acids are different. The OH bond length and IR redshift afford the better measure of hydrogen bond strength.