High-resolution synchrotron terahertz investigation of the large-amplitude hydrogen bond librational band of (HCN)₂

Abstract

The high-resolution terahertz absorption spectrum of the large-amplitude intermolecular donor librational band ν of the homodimer (HCN) has been recorded by means of long-path static gas-phase Fourier transform spectroscopy at 207 K employing a highly brilliant electron storage ring source. The rovibrational structure of the ν band has the typical appearance of a perpendicular type band of a Σ-Π transition for a linear polyatomic molecule. The generated terahertz spectrum is analyzed employing a standard semi-rigid linear molecule Hamiltonian, yielding a band origin ν of 119.11526(60) cm together with values for the excited state rotational constant B', the excited state quartic centrifugal distortion constant D' and the l-type doubling constant q for the degenerate state associated with the ν mode. The until now missing donor librational band origin enables the determination of an accurate experimental value for the vibrational zero-point energy of 2.50 ± 0.05 kJ mol arising from the entire class of large-amplitude intermolecular modes. The spectroscopic findings are complemented by CCSD(T)-F12b/aug-cc-pV5Z (electronic energies) and CCSD(T)-F12b/aug-cc-pVQZ (force fields) electronic structure calculations, providing a (semi)-experimental value of 17.20 ± 0.20 kJ mol for the dissociation energy D of this strictly linear weak intermolecular CHN hydrogen bond.

Fundamental Vibrational Analyses of the HCN Monomer, Dimer and Associated Isotopologues

“…Mihrin and coworkers [2] very recently observed the n 8 band via terahertz spectroscopy utilizing synchrotron radiation. Mihrin and coworkers [2] very recently observed the n 8 band via terahertz spectroscopy utilizing synchrotron radiation.…”

“…Mihrin and coworkers [2] also did a high-level theoretical workup of the vibrational spectrum of the HCN dimer species, though they did not go beyond the harmonic approximation. They report frequencies at the CCSD(T)-F12b explicity correlated level of theory while utilizing an augmented quadruple-z quality basis set (aug-cc-pVQZ).…”

“…Refer to Table IV equilibrium monomer separation. The latest value from Mihrin and coworkers [2] for the energy of dimer formation is a so-called "semi-experimental" result which uses the CCSD(T)-F12b/augcc-pV5Z association energy corrected by the zero-point vibrational energy of experimental frequencies. [25] This too, shows fair agreement with our theoretical results.…”

“…[17] Several groups employing microwave spectroscopy [23][24][25][26] have determined the rotational r 0 structure and rotational constants of the parent and substituted isotopologues of the dimer. Several IR studies, utilizing matrix isolation, [27][28][29][30][31][32][33] gas phase, [2,19,[34][35][36][37][38][39] and super-fluid helium, [20] have also added to the wealth of experimental results available for this dimer system. These experimental findings arecomplimented by many theoretical studies.…”

“…The most recent study of the dimer system, published by Mihrin and co-workers, [2] studied the dimer via Fourier transform terahertz spectroscopy. Utilizing this method, they were able to observe, to a very high precision, the fundamental vibrational frequency, n 8 which they refer to as the "donor libration" vibrational mode.…”