Vibrational States and Nitrile Lifetimes of Cyanophenylalanine Isotopomers in Solution

Abstract

Nitrile lifetimes and the structure of the vibrational state space of four isotopomers of cyanophenylalanine in solution are calculated. While the frequency of the nitrile of the four isotopomers decreases in the order CN, CN, CN, and CN, the lifetime varies nonmonotonically with the change in frequency. The vibrational properties of the molecules that control the lifetime are examined. The specific resonances that contribute to the lifetime are tuned by isotopic substitution, and the magnitudes of the anharmonic constants involved in the coupling of vibrations that mediate the lifetime of the nitrile vary with CN mass. The nature of the modes coupled to the nitrile varies, as the frequency of the nitrile changes with isotopic substitution. For some CN frequencies the modes coupled to the CN are rather localized to the ring, while at other frequencies the modes coupled to the CN are more delocalized. Comparison of the calculated frequencies and lifetimes with recent experimental measurements on these molecules is discussed.

Electric Fields Influence Intramolecular Vibrational Energy Relaxation and Line Widths

“…This is due to alteration of the vibrational energy transfer rate from nitrile to other overtones of the low frequency modes. 20 This intramolecular energy dissipation phenomena for the CN mode has been observed in isotopically substituted nitriles. Here, the perturbing factor that changes the intramolecular relaxation is mass.…”

“…Recent experimental and theoretical studies have shown that the nitrile frequency-shift changes the nitrile vibrational lifetime due to intramolecular coupling with other low frequency modes present in the molecule. This is due to alteration of the vibrational energy transfer rate from nitrile to other overtones of the low frequency modes . This intramolecular energy dissipation phenomena for the CN mode has been observed in isotopically substituted nitriles.…”

“…19 As the electronic potential of the molecule does not change due to isotopic substitution, the contributing factor to the nitrile lifetime was due to the intramolecular vibrational relaxation. 20 2D IR experiments by Tucker et al showed that by tuning the intramolecular coupling it is possible to tune the vibrational energy flow in p-azidobenzonitrile (PAB) and p-(azidomethyl)benzonitrile (PAMB). 26 Outside of the context of Stark shift spectroscopy, long-lived vibrations are important in mediating chemical reactions in gas and condensed phase and in biomolecules.…”

“…First, the molecule reports on the fluctuations and inhomogeneities of the environment as an antenna, and thereby encodes the environmental variations in its line width. Second, the Stark-active vibration is often coupled to other low frequency modes within the same molecule and can dissipate energy into those modes thereby contributing to the line width. , When the second mechanism is operative, interpreting the line widths as merely a representation of the environmental fluctuations via an antenna effect becomes problematic. In this work, we show that not only there is a significant intramolecular source of line broadening in the benzonitrile Stark probe molecule, but also that such intramolecular pathways of energy dissipation change as a function of applied electric field.…”

“…IR pump–probe measurements of isotopically substituted p -cyanophenylalanine (Phe-CN) by Rodgers et al revealed a nonmonotonic lifetime trend with respect to reduced mass . As the electronic potential of the molecule does not change due to isotopic substitution, the contributing factor to the nitrile lifetime was due to the intramolecular vibrational relaxation . 2D IR experiments by Tucker et al showed that by tuning the intramolecular coupling it is possible to tune the vibrational energy flow in p -azidobenzonitrile (PAB) and p -(azidomethyl)­benzonitrile (PAMB) …”