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  • DFT-Based Calculation of Molecular Hyperpolarizability and SFG Intensity of Symmetric and Asymmetric Stretch Modes of Alkyl Groups
Title: DFT-Based Calculation of Molecular Hyperpolarizability and SFG Intensity of Symmetric and Asymmetric Stretch Modes of Alkyl Groups
Vibrational sum frequency generation (SFG) spectroscopy has been extensively used for obtaining structural information of molecular functional groups at two-dimensional (2D) interfaces buried in the gas or liquid medium. Although the SFG experiment can be done elegantly, interpreting the measured intensity in terms of molecular orientation with respect to the lab coordinate is quite complicated. One of the main reasons is the difficulty of determining the hyperpolarizability tensors of even simple molecules that govern their SFG responses. The single-bond polarizability derivative (SBPD) model has been proposed to estimate the relative magnitude of SFG-active hyperpolarizability by assuming that the perturbation associated to each vibration is negligible. In this study, density functional theory (DFT) was used to calculate the polarizability and dipole derivative tensors of the CH3 stretch mode of CH3I, CH3¬CH2I, CH3OH, CH3¬CH2OH. Then, the hyperpolarizability tensors of symmetric and asymmetric vibration modes were calculated considering the Boltzmann distribution of representative conformers, which allowed theoretical calculation of their SFG intensities at all polarization combinations as a function of the tilt angle of the CH3 group with respect to the surface normal direction. Then the ratios of the calculated SFG intensities for the CH3 peaks used in experimental studies for the CH3 tilt angle determination were compared. This comparison clearly showed the effect of vibrational coupling among neighboring functional groups. This study presents new parameters that can be used in determining the average tilt angle of the CH3 group at the 2D interface with SFG measurements as well as limitations of the method.  more » « less
Award ID(s):
2203635
PAR ID:
10515087
Author(s) / Creator(s):
; ; ;
Publisher / Repository:
American Chemical Society
Date Published:
Journal Name:
The Journal of Physical Chemistry B
Volume:
127
Issue:
39
ISSN:
1520-6106
Page Range / eLocation ID:
8456 to 8467
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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