OBSERVATIONS OF ROTATIONAL MAGNETIC-MOMENTS IN THE GROUND AND SOME EXCITED VIBRATIONAL SIGMA-STATES OF C2H2, C2HD, AND C2D2 BY MAGNETIC VIBRATIONAL CIRCULAR-DICHROISM

Magnetic vibrational circular dichroism (MVCD) spectra of acetylene an d its deuterated isotopomers have been recorded for the following Sigm a symmetry combination and overtone bands of C2H2: nu(4)+nu(5); C2HD: nu(4)+nu(5); 2 nu(4), 2 nu(5); C2D2: nu(4)+nu(5): the nu(3) fundamenta l for C2HD and C2D2; and the nu(4)-->2 nu(4)+nu 5 and nu(5)-->nu(4)+2 nu(5) hot bands for C2H2. For a Sigma(g)-->Sigma(u) transition, the MV CD A terms observed must arise primarily from the rotational Zeeman ef fect. These negative A(1)/D-0 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 s tates were determined by comparison of moment analyses of experimental and simulated MVCD spectra obtained with a model Hamiltonian for acet ylene. The g(J) values in all the excited bending combination and over tone 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 th e nu(4)+nu(5) combination band of C2H2. From these MVCD results, for C 2H2, g(J)(ground) = +0.0535+/-0.0033 and Delta g(nu(4)+nu(5)) = -0.006 1+/-0.0004; for C2HD, g(J)(ground)similar to g(J)(nu(3)) = +0.0363+/-0 .0048, Delta g(nu(4)+nu(5)) = -0.0052+/-0.0031; and for C2HD, g(J)(gro und)similar to g(J)(nu(3)) = +0.0409 +/- 0.0069. These are the first q uantitative, MVCD determinations of nondegenerate excited state g valu es distinctly different from the ground state. The decrease in g value correlates with off-axis deformation of the linear C2H2 rotation. (C) 1996 American Institute of Physics.