Relaxation-Assisted 2D IR Using Weak Vibrational Modes

Abstract

Implementation of weak IR modes as structural reporters via dual-frequency 2D IR spectroscopy is demonstrated. Prominent cross peaks originated from interactions of C≡N with C=O and C≡N with C=C modes were measured in 3-cyanocoumarin using heterodyned three-pulse photon-echo technique. In 4-acetylbenzonitrile, where the distance between CN and CO groups is over two times larger than in 3-cyanocoumarin, the direct coupling was just below the level of detectivity. A new method of measuring 2D spectra is proposed that utilizes vibrational relaxation phenomena. Using the new method the cross peaks at (ω_CN, ω_CO) has easily been detected; over six times increase of the cross-peak amplitude has been observed when the waiting time between the second and third pulses, T, was varied. Calculations show that using this approach vibrational modes separated by over 7–10 Å can be used as structural probes enhancing substantially detectable distances via 2D IR, especially when weak IR modes are used. We have also demonstrated implementation of C-D stretching modes as structural reporters. Although C-D modes have very small transition dipoles, interaction with C≡N mode has been cleanly detected in acetonitrile-D3. As C-H groups are widespread in organic molecules, the deuterium can be placed in the vicinity of virtually any group. Implementation of new labels, reinforced by vibrational-relaxation-assisted 2D IR method, provides universal labeling strategies for structure determination via 2D IR.