Anharmonic coupling of the CH-stretch and CH-bend vibrations of chloroform as studied by near-infrared electroabsorption spectroscopy

Abstract

Combination bands that involve CH- or OH-stretch vibrations appear in the near-infrared (NIR) region (4000-10 000 cm(-1)). Because they arise from anharmonic coupling between the component fundamentals, detailed analysis of the frequency and intensity of NIR combination bands allows one to elucidate the mechanisms behind the vibrational coupling in the condensed phase in terms of mechanical and electrical anharmonicities. Nevertheless, little has been studied, in particular experimentally, on the origin of the combination band intensity. Here, we show that NIR electroabsorption (EA) spectroscopy, which directly probes the effects of an externally applied electric field on a combination band, can shed new light on anharmonic vibrational coupling through determination of the direction of the transition moment for the combination band. We studied the combination band of the CH-stretch (ν(1)) and CH-bend (ν(4)) modes of liquid chloroform. The electric-field induced absorbance change of the ν(1) + ν(4) combination band caused by reorientation of the chloroform molecule was measured at various χ angles, where χ is the angle between the direction of the applied electric field and the polarization of the incident IR light. We were able to detect an absorbance change as small as 5 × 10(-8) for the combination band. Using the NIR EA spectra of the combination band together with those of the CH-stretch and bend fundamentals, the angle between the transition moment for the combination band and the permanent dipole moment was determined experimentally for the first time to be (79 ± 14)°. The present investigation indicates that the contribution of the CH-stretch mode to the mechanical anharmonicity is minor and that the CH-bend mode plays a dominant role in the mechanical part of the vibrational coupling between the two fundamentals. Furthermore, density functional theory calculations show that both the mechanical anharmonicity of the CH-bend mode and the electrical anharmonicity may contribute equally to the anharmonic coupling.

Mode coupling and multiquantum vibrational excitations in Feshbach-resonant positron annihilation in molecules

“…A detailed analysis of the escape width shows that the largest contribution to it comes from the ν 1 → ν 4 transition, which gives 87% of the width Γ v ν . The anharmonic coupling between the CH-stretch (ν 1 ) and CH-bend (ν 4 ) was investigated earlier by observing the ν 1 +ν 4 combination band in near-infrared absorption and supported by density-functional calculations [47]. Suppression of the CH-stretch VFR due to inelastic escape was also inferred empirically in previous positron annihilation studies of chloroform and chloroform-d 1 [21,22] and in fluoroalkanes [1,14,43].…”

Organic Thin Films Based on DPP-DTT:C60 Blends Deposited by MAPLE

“…If the characteristic vibrations of the conjugated polymer are clearly identified in the FTIR spectra of the DPP-DTT:C60 thin films deposited by MAPLE, the typical vibrations of C60 located at about 525, 574, and 1430 cm −1 [43] can be barely noticed in these spectra due to their weak intensity. Additionally, in the FTIR spectra of the investigated samples the appearance of the vibrations assigned to the chloroform molecules situated at about 1215 and 3019 cm −1 related to the C-H stretching and C-H bending, respectively [44], is due to the presence of traces of this solvent in the organic layers deposited by MAPLE. In a molecular dynamic simulation study regarding the ejection and transport of organic molecules in the MAPLE process, it was evidenced that some droplets on the substrate may not be completely free of solvent molecules, resulting in the deposition of organic films with solvent traces [45].…”

Isotope Effects on the Vaporization of Organic Compounds from an Aqueous Solution–Insight from Experiment and Computations

“…However, if a complex is formed (or hydrogen bonds form), the number of vibrational modes increases, and the intermolecular vibrations, in particular, those originating from directed hydrogen bonds, will be additionally coupled to the C–H stretching mode. There is yet another vibrational mode that involves the motion of the hydrogen atom—the C–H bending mode, which along with the C–H stretching modes gives a combination mode arising from anharmonic coupling (see ref ( 70 ) and refs therein). These combination bands appear in the near-IR region, which is usually not covered in a normal mode analysis performed to compute isotope effects.…”