Observations of rotational magnetic moments in the ground and some excited vibrational Σ states of C2H2, C2HD, and C2D2 by magnetic vibrational circular dichroism

Abstract

Magnetic vibrational circular dichroism (MVCD) spectra of acetylene and its deuterated isotopomers have been recorded for the following Σ symmetry combination and overtone bands of C2H2: ν4+ν5; C2HD: ν4+ν5, 2ν4, 2ν5; C2D2: ν4+ν5, the ν3 fundamental for C2HD and C2D2; and the ν4→2ν4+ν5 and ν5→ν4+2ν5 hot bands for C2H2. For a Σg→Σu transition, the MVCD A terms observed must arise primarily from the rotational Zeeman effect. These negative A1/D0 values for low J″ transitions confirm that the sign of the rotational g-value for acetylene is positive. The rotational magnetic moments in both the lower and upper vibrational states were determined by comparison of moment analyses of experimental and simulated MVCD spectra obtained with a model Hamiltonian for acetylene. The gJ values in all the excited bending combination and overtone vibrational levels observed are smaller than those in the ground and the first excited stretching vibrational levels. This observation has been confirmed by theoretical simulation of the MVCD spectra of the ν4+ν5 combination band of C2H2. From these MVCD results, for C2H2, gJ(ground)=+0.0535±0.0033 and Δg(ν4+ν5)=−0.0061±0.0004; for C2D2, gJ(ground)∼gJ(ν3) =+0.0363±0.0048, Δg(ν4+ν5)=−0.0052±0.0031; and for C2HD, gJ(ground)∼gJ(ν3)=+0.0409 ±0.0069. These are the first quantitative, MVCD determinations of nondegenerate excited state g values distinctly different from the ground state. The decrease in g value correlates with off-axis deformation of the linear C2H2 rotation.