Molecular Structure and Photoinduced Intramolecular Hydrogen Bonding in 2‑Pyrrolylmethylidene Cycloalkanones

The structures of pyrrolylmethylidene derivatives of 2,3-dihydro-1H-inden-1-one (3), 3,4-dihydro-naphthalen-1­(2H)-one (4), and cycloalkanones (5–7) were studied for the first time in the solid state and solution by NMR, IR, and UV spectroscopies supported by DFT quantum mechanical calculations. It was shown that all studied compounds except cycloheptanone derivative 7 both in crystal and in solution exist in the form of dimers where single E or E,E configuration with respect to the exocyclic CC bond is stabilized by intermolecular hydrogen bonds N–H···OC. UV irradiation at a wavelength of 365 nm of MeCN or DMSO solutions of 3–6 results, depending on the exposition time and solvent, partial to complete isomerization to the Z or Z,E isomers (in the case of 6, also the Z,Z isomer). The NMR and IR spectroscopy data show the existence of a strong intramolecular hydrogen bond N–H···OC in the Z moieties of isomerized compounds. The studied compounds are protonated by trifluoroacetic acid at the carbonyl oxygen, in spite of the reverse order of basicity and nucleophilicity of the carbonyl group and the pyrrole ring. Investigation of the behavior of compound 6 with respect to acetate and fluoride anions allows one to consider it as a potential fluoride sensor.本研究首次结合密度泛函理论（DFT）量子力学计算，采用核磁共振（NMR）、红外（IR）与紫外（UV）光谱技术，对2,3-二氢-1H-茚-1-酮（3）、3,4-二氢-1(2H)-萘酮（4）以及环烷酮（5~7）的吡咯基亚甲基（pyrrolylmethylidene）衍生物的固态与溶液态结构展开了系统研究。结果显示，除环庚酮衍生物7外，所有受试化合物在晶体及溶液中均以二聚体形式存在，其外环C=C双键的单一E或E,E构型可通过分子间氢键N–H···O=C得以稳定。对3~6的乙腈（MeCN）或二甲基亚砜（DMSO）溶液施加365 nm波长的紫外辐照后，异构化程度取决于辐照时长与溶剂类型，可实现从部分到完全的异构转化，生成Z或Z,E异构体（对于化合物6，还可得到Z,Z异构体）。核磁共振与红外光谱数据表明，异构化产物的Z构型片段中存在强分子内氢键N–H···O=C。尽管羰基与吡咯环的碱性及亲核性顺序存在反转，但受试化合物仍会在羰基氧位点被三氟乙酸（trifluoroacetic acid）质子化。通过考察化合物6与乙酸根（acetate）及氟离子（fluoride anions）的相互作用行为，可将其视为一种潜在的氟离子传感器。