The microwave spectrum of 3,4-dimethylfluorobenzene was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz with the goal of quantifying steric effects on barriers to internal rotation of the two inequivalent methyl groups. Due to these torsional motions, splittings of all rotational transitions into quintets were observed and fitted with residuals close to measurement accuracy. The experimental work was supported by quantum chemical calculations, and the B3LYP-D3BJ/6-311++G(d,p) level of theory yielded accurate optimized geometry parameters to guide the assignment. The three-fold potential values of 456.20(13) cm⁻¹ and 489.78(15) cm⁻¹ for the methyl groups at the meta and para position, respectively, deduced from the experiments are compared with the predicted values and those of other toluene derivatives.

3,4-二甲基氟苯的微波光谱是使用脉冲分子射流傅里叶变换微波光谱仪在2.0至26.5 GHz的频率范围内测量的，目的是量化对两个不等价甲基的内部旋转壁垒的空间效应。由于这些扭转运动，观察到所有旋转过渡都分裂成五重峰，并拟合了接近测量精度的残差。实验工作得到了量子化学计算的支持，并且B3LYP-D3BJ / 6-311 ++ G（d，p）的理论水平产生了精确的优化几何参数来指导分配。甲基和间位甲基的三倍电位值分别为456.20（13）cm ^-1和489.78（15）cm ^-1将实验得出的对位与预测值和其他甲苯衍生物的预测值进行比较。