C. Castiglioni

Infrared intensities. Use of the CH-stretching band intensity as a tool for evaluating the acidity of hydrogen atoms in hydrocarbons

Abstract The accurate study of the infrared spectra (both frequencies and intensities) of a large number of molecules and the decodification of their intensities in terms of parameters representing changes of charge distribution with vibration have provided standards that, at least in the case of molecules with an appreciable presence of CH bonds, may now be used to give information on the acidity of the hydrogen atoms even based only on the qualitative aspects of the infrared spectrum in the CH-stretching region. The charge on the hydrogen atom of a CH bond may change by hybridization of the carbon atom, by induction of polarization in the CH bond by a close electronegative atom, by backdonation into the CH bond of negative charge from any lone pair of an electronegative atom and with hyperconjugation of CH3 and CH2 with the electrons of double and triple bonds. The variation of the hydrogen charge is also accompanied by a change in the CH stretching frequencies, as reported in several works by McKean and coworkers. It has to be pointed out that the change in the CH stretching intensities is almost always more noticeable and often so large that it can be detected by the simple qualitative examination of the spectrum. We present here examples of such a qualitative analysis.

Relaxation contribution to hyperpolarizability. A semiclassical model

Abstract In the presence of strong electromagnetic fields, the response of the material may be non linear. This is usually quantified in terms of the α, β, γ,…tensors. A non-negligible contribution to these tensors originates from the relaxation of nuclei. In this paper a semiclassical treatment is presented which allows to derive an explicit analitic expression for this contribution in terms of vibrational spectroscopic observables.

Charge distribution from infrared intensities: Charges on hydrogen atoms and hydrogen bond

It is known that from infrared intensities it is possible to derive atomic charges and charge fluxes. The intensity parameters of hydrogen atoms which are able to form hydrogen bonds are calculated in this work and compared with those of other hydrogen atoms not showing the same property; they are found to be different. From gas phase infrared intensity measurements one can then determine whether a given hydrogen atom is capable of forming hydrogen bond.

Characteristic infrared intensities of CH bonds

Abstract The contribution for CH bond to the total intensity in the CH stretching region (Astr)and to the total intensity in the CH deformation region (Adef) can be easily measured for several compounds. Astr and Adef provide a direct information on the electrical properties of the CH bonds in different surroundings.

Non-linear optical response to strong applied electromagnetic fields in polyconjugated materials

Abstract Recent applications in the field of photonics require materials with a large non-linear response to applied fields. Among the tools that can indicate which organic material has a large hyperpolarizability, there is vibrational spectroscopy. It is shown here that materials with very large Raman intensities have a large, second hyperpolarizability accounted for in terms of relaxation mechanism; it is also suggested that doped materials, which have an anomalously large infrared intensity, can have a large, first hyperpolarizability. Moreover, the relaxation mechanism involved in hyperpolarizabilities can be explained in terms of changes in geometries along the direction of the “effective conjugation coordinate”Я

Ab initio counterpart of infrared atomic charges. Comparison with charges obtained from electrostatic potentials

Abstract Atomic charges obtained from infrared intensities have already been shown to be adequately represented by Mulliken net charges and even better by a version of Mulliken charges which makes them fit the molecular dipole moment. It is shown in this paper that the atomic charges derived from electrostatic potentials give the same picture of the charge distribution within molecules as that obtained from the corrected Mulliken charges.

Vibrational Spectroscopy of Polyconjugated Aromatic Materials with Electrical and Non Linear Optical Properties

The fast development of modern material science is based on the existence of many physical techniques which are used for the characterisation of the molecular and/or bulk properties of the materials. The data so collected form the basis of any attempt to understand on a theoretical basis the physics of the phenomena which originate the properties of the material.

Experimental atomic charges from infrared intensities. Comparison with “ab initio” values

Abstract Atomic charges can be derived from observed infrared intensities and molecular dipole moments. The atomic charges so drived for a series of simple organic molecules are compared with atomic electron population data computed by quantum-mechanical calculations at various levels. It is shown that experimental charges agree very well with those computed by 6-31G**.

Charge mobility in σ‐bonded molecules: The infrared spectrum of polymethylene chains in the solid and liquid phases

The experimental observation we wish to explain and which is common in most of the polymethylene systems with no polar substituents is that the absorption intensity in the infrared of the CH2 wagging motions is extremely weak for solid all‐trans chains and becomes much stronger in the liquid phase in which molecules are conformationally distorted. Using current theories for the interpretation of the infrared intensities, it is shown that the observed phenomenon arises from the fact that for all transplanar chains, a sizeable ‘‘σ conjugation’’ along the CC chain takes place. It is shown that ‘‘charge fluxes’’ from second nearest neighbors play the main role in determining such intensity behavior.

Peierls distortion in trans polyacetylene: evidence from infrared intensities.

Abstract It is shown that from the polarization properties of the infrared spectrum of oriented pristine trans polyacetylene clear evidence of Peierls distortion is obtained. Moreover, it is shown that during CH stretching and in-plane bending CC bonds develop relevant charge fluxes. Infrared intensity studies indicate the existence of an unusual charge mobility.