Atomistic Modeling of IR Action Spectra Under Circularly Polarized Electromagnetic Fields: Toward Action VCD Spectra

Abstract

The nonlinear response and dissociation propensity of an isolated chiral molecule, camphor, to a circularly polarized infrared laser pulse was simulated by molecular dynamics as a function of the excitation wavelength. The results indicate similarities with linear absorption spectra, but also differences that are ascribable to dynamical anharmonic effects. Comparing the responses between left- and right-circularly polarized pulses in terms of dissociation probabilities, or equivalently between R- and S-camphor to a similarly polarized pulse, we find significant differences for the fingerprint C = O amide mode, with a sensitivity that could be sufficient to possibly enable vibrational circular dichroism as an action technique for probing molecular chirality and absolute conformations in the gas phase.

Vibrational Circular Dichroism Spectroscopy with a Classical Polarizable Force Field: Alanine in the Gas and Condensed Phases

“…In addition to the electric dipole moment $${A = \vec \mu }$$ or its derivative $${{\dot {\vec \mu}} }$$ , the VCD signal requires, for C AB , the magnetic dipole moment $${B = {\vec m}}$$ that characterizes the rotation of the electric current around the axes of propagation, characteristic of circularly polarized light, to be considered. The resulting expression for the VCD intensity reads: [14,19,41,42] $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {\rm{\Delta }}A_{\dot \mu } (\omega ) = {{8\pi \beta \omega } \over {3Vcn(\omega )}}\int_{ - \infty }^{ + \infty } {\left\langle {{\dot {\vec \mu}} (0) \cdot {\vec m}(\tau )} \right\rangle } e^{ - i\omega \tau } d\tau .\hfill\cr}}$$$ …”

“…[1] In addition to the electric dipole moment A ¼ m or its derivative _ m, the VCD signal requires, for C AB , the magnetic dipole moment B ¼ m that characterizes the rotation of the electric current around the axes of propagation, characteristic of circularly polarized light, to be considered. The resulting expression for the VCD intensity reads: [14,19,41,42] DA _ m ðwÞ ¼ 8pbw 3VcnðwÞ…”

“…[21,22] Alternatively, polarizable force fields have become sufficiently reliable to allow for the realistic modeling of entire infrared spectra, both in the gas [34][35][36][37] and condensed [34,35,[38][39][40] phases. However, so far only nonpolarizable FFs have been employed to determine VCD spectra, [19,41,42] whereas suitably parametrized polarizable FFs can handle both large systems, long sampling times, still remaining chemically accurate. The purpose of the present article is to extend the framework of polarizable force fields to the calculation of VCD spectra.…”

“…[19,41] Additional terms arise for a polarizable system, for which the induced electric dipole moment can also contribute to the magnetic dipole moment. The methodology proposed here extends the classical treatment of magnetic dipole moments [19,41,42] to include several contributions arising from the induced electric dipole moment and its time derivative, as inspired from earlier work based on the nuclear velocity perturbation theory approach to VCD spectroscopy. [31] As the scheme we develop is meant to be accurate, it is important to benchmark the electric and magnetic dipole moments against calculations performed at a higher level of theory.…”

Vibrational circular dichroism spectroscopy with a classical polarizable force field: alanine in the gas and condensed phases

“…[12,13] Alternatively, polarizable force fields have become sufficiently reliable to allow for the realistic modeling of entire infrared spectra, both in the gas [25][26][27][28] and condensed [25,26,[29][30][31] phases. However, so far only nonpolarizable FFs have been employed to determine VCD spectra, [10,32,33] whereas suitably parametrized polarizable FFs can handle both large systems, long sampling times, still remaining chemically accurate. The purpose of the present article is to extend the framework of polarizable force fields to the calculation of VCD spectra.…”