Abstract The rotational spectrum of 3-furfural (3-FF) was investigated using Fourier transform microwave spectroscopy from 6 to 19 GHz in order to examine the rotational isomerism properties of the formyl group relative to the furan ring backbone. Two planar rotamers with the formyl group anti or syn to the ring oxygen atom were observed with the latter being identified in the gas phase for the first time. The observed relative intensity of ∼8:1 (anti:syn) of transitions is consistent with quantum chemical calculations at the B3LYP-D3(BJ)/aug-cc-pVTZ level which predict that the anti rotamer is more stable than the syn by 4.86 kJ/mol. Transitions due to singly-substituted 13C isotopologues for both rotamers and the 18O isotopologues for the anti form were assigned. The resulting sets of rotational constants allowed derivation of accurate ground state effective (r0) and mass dependent (rm(1)) geometries for the furan ring of each rotamer for comparison with their equilibrium (re) geometries. The syn/anti arrangement of the formyl group appears to have little effect on the geometry of the furan ring itself but the exocyclic group is rotated slightly closer to the oxygen atom of the heterocycle when in the anti arrangement.

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3-糠醛旋转异构体的微波光谱研究

摘要 为了检测甲酰基相对于呋喃环骨架的旋转异构特性，使用傅立叶变换微波光谱在 6 至 19 GHz 范围内研究了 3-糠醛 (3-FF) 的旋转光谱。观察到两个平面旋转异构体，其甲酰基与环氧原子反或顺，后者首次在气相中被鉴定。观察到的~8:1 (anti:syn) 跃迁的相对强度与 B3LYP-D3(BJ)/aug-cc-pVTZ 水平的量子化学计算一致，预测反旋转异构体比 syn 4.86 千焦/摩尔。由于旋转异构体的单取代 13C 同位素体和反形式的 18O 同位素体导致的转变被指定。由此产生的旋转常数组允许推导出每个旋转异构体的呋喃环的准确基态有效 (r0) 和质量相关 (rm(1)) 几何形状，以便与它们的平衡 (re) 几何形状进行比较。甲酰基的顺/反排列似乎对呋喃环本身的几何形状几乎没有影响，但在反排列时，环外基团旋转得稍微靠近杂环的氧原子。