Direct measurement of the rotational g-value in the ground vibrational state of acetylene by magnetic vibrational circular dichroism

The magnetic vibrational circular dichroism (MVCD) spectra of the v4 + v5 combination band of acetylene was measured. from the result for the P(J″ = 1 → J′ = 0) rotational transition, the gJ-value in the ground vibrational state for C2H2 was determined to be +0.049 (±0.005). The sign of the gJ-value measured by MVCD confirms that the correct sign is positive as found in a revised molecular beam magnetic resonance experiment and predicted on the basis of molecular Zeeman studies of other molecules and magnetic effects on heat flow in acetylene. MVCD offers an exceptionally simple method of determining the sign of molecular g-values for both the ground and excited states.