Revisiting the IR Spectra of H₂S in an Argon Matrix: Identification of (H₂S) (n ≤ 4) Clusters via the Spectral Assignment of ν_S-H Transitions

Small-molecular clusters of H₂S up to tetramer have been experimentally identified in a cold and solid argon matrix by the spectral assignment of ν_S-H fundamental transitions for different H₂S:argon mixing ratios. Normal-mode frequency calculations at the MP2-CP/aug-cc-pV(Q + d)Z level have been used to support the spectral assignments. In addition, modulations in relative populations of different clusters due to the annealing of the deposited matrix and the preheating of the H₂S-argon gas mixture before deposition reinforced the spectral assignments. Variations in mixing ratio, annealing of the matrix, and preheating of the gas mixture have also been used, in a combined manner, to unambiguously identify the ν₁ and ν₃ bands of the H₂S monomer, which has been a matter of dispute for a long period. The two bands have been identified at 2634.4 and 2648.0 cm^-1, respectively, while three bands at 2581.5, 2568.4, and 2547.6 cm^-1 have been assigned to H-bonded dimers, cyclic trimers, and cyclic tetramers, respectively. Multiple bands within 2550-2580 cm^-1 have been assigned to caged tetramers. The cooperative strengthening of S-H···S H bonds in cyclic H₂S clusters was evident from the linear increment in ν_S-H spectral shifts with cluster size.