Vibrational couplings and energy flow in complexes of NH3 with HCN, HCCH, and HCCCCH

Hilpert, G.; Fraser, G. T.; Pine, A. S.
October 1996
Journal of Chemical Physics;10/15/1996, Vol. 105 Issue 15, p6183
Academic Journal
A tunable color-center laser and a molecular-beam electric-resonance optothermal spectrometer have been used to record the infrared spectra of the C–H stretching vibrations of HCCH–NH3, HCCD–NH3, NCH–NH3, and HCCCCH–NH3. The hydrogen-bonded C–H stretching vibrations of NCH–NH3, HCCH–NH3, and HCCCCH–NH3 are redshifted by 200.88126(30), 75.1042(38), and 127.4(1) cm-1 from the respective free monomer modes. The non-hydrogen-bonded C–H stretches are less perturbed by complexation, being blueshifted by 0.2992(3) cm-1 in HCCCCH–NH3 and redshifted by 1.179(1) cm-1 in HCCD–NH3. Consistent with the much larger perturbation of the monomer vibration for the bonded C–H stretch, the B rotational constants increase by 1%–2% for the bonded C–H stretch excited NCH–NH3 and HCCH–NH3 complexes, but change by less than 0.1% for the nonbonded C–H stretches in HCCCCH–NH3 and HCCD–NH3. The decoupling of the two C–H stretches in HCCH–NH3 is not sufficient to allow the observation of the nonbonded C–H stretch in the complex, which correlates to the Raman-active symmetric C–H stretch of acetylene. Also, no spectra were observed for the weaker N–H stretching vibrations of the complexes, consistent with the very weak intensities of these modes in the monomer. The homogeneous linewidths of the transitions, assumed to be a measure of the vibrational predissociation rate, are approximately two orders of magnitude larger for the bonded C–H stretches than for the nonbonded C–H stretches. The similarity in homogeneous widths for the nonbonded C–H stretches in HCCD–NH3 and HCCCCH–NH3, of 7–12 MHz, suggests that the rate of vibrational energy flow along acetylene chains is only weakly dependent on chain length.


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