Lee Giseong, Kossowska Dorota, Lim Joonhyung, Kim Soobin, Han Hogyu, Kwak Kyungwon, Cho Minhaeng
Department of Chemistry , Korea University , Seoul 02841 , Korea.
Center for Molecular Spectroscopy and Dynamics , Institute for Basic Science (IBS) , Seoul 02841 , Korea.
J Phys Chem B. 2018 Apr 12;122(14):4035-4044. doi: 10.1021/acs.jpcb.8b00887. Epub 2018 Mar 22.
Infrared (IR) probes based on terminally blocked β-cyanamidoalanine (AlaNHCN) 1 and p-cyanamidophenylalanine (PheNHCN) 2 were synthesized, and the vibrational properties of their CN stretch modes were studied using Fourier transform infrared (FTIR) and femtosecond IR pump-probe spectroscopies in combination with quantum chemical calculations. From FTIR studies, it is found that the transition dipole strengths of the cyanamide (NHCN) group in 1 and 2 are much larger than those of the nitrile (CN) group but comparable to those of the isonitrile (NC) and azido (N) groups in their previously studied analogs. The CN stretch frequencies in 1 and 2 are red-shifted from those in their nitrile analogs but more blue-shifted from the NC and N stretch frequencies in their isonitrile and azido analogs. The much larger transition dipole strength and the red-shifted frequency of the cyanamide relative to nitrile group originates from the n → π* interaction between the N atom's nonbonding (n) and CN group's antibonding (π*) orbitals of the NHCN group. Unlike aliphatic cyanamide 1, aromatic cyanamide 2 shows a complicated line shape of the CN stretch spectra. Such a complicated line shape arises from the Fermi resonance between the CN stretch mode of the NHCN group and one of the overtones of the phenyl ring vibrations and can be substantially simplified by deuteration of the NHCN into NDCN group. From IR pump-probe experiments, the vibrational lifetimes of the CN stretch mode in 1 were determined to be 0.58 ± 0.04 ps in DO and 0.89 ± 0.09 ps in HO and those in 2 were determined to be 1.64 ± 0.13 ps in CHOD/dimethyl sulfoxide and 0.30 ± 0.05 and 2.62 ± 0.26 ps in CHOH. The short time component (0.30 ± 0.05 ps) observed for 2 in CHOH is attributed to the vibrational relaxation through Fermi resonance. These vibrational lifetimes are close to those of the nitrile and azido groups but shorter than those of the isonitrile group. Consequently, cyanamide behaves like an apparent vibrational hybrid of nitrile and isonitrile in that cyanamide is similar to nitrile in vibrational frequency and lifetime but to isonitrile in transition dipole strength. It is believed that cyanamide has the potential to be a strongly absorbing IR reporter of the conformational and environmental structure and dynamics of biomolecules in comparison to nitrile, a weak absorber.
合成了基于末端封端的β-氰基氨基丙氨酸（AlaNHCN）1和对氰基氨基苯丙氨酸（PheNHCN）2的红外（IR）探针，并结合量子化学计算，使用傅里叶变换红外（FTIR）和飞秒红外泵浦-探测光谱研究了它们的CN伸缩模式的振动特性。通过FTIR研究发现，1和2中氰胺基（NHCN）的跃迁偶极强度远大于腈基（CN），但与之前研究的类似物中的异腈基（NC）和叠氮基（N）相当。1和2中的CN伸缩频率相对于腈类类似物发生红移，但相对于异腈和叠氮类似物中的NC和N伸缩频率发生更多蓝移。氰胺基相对于腈基更大的跃迁偶极强度和红移频率源于NHCN基团中N原子的非键（n）轨道与CN基团的反键（π*）轨道之间的n→π*相互作用。与脂肪族氰胺1不同，芳香族氰胺2的CN伸缩光谱呈现复杂的线形。这种复杂的线形源于NHCN基团的CN伸缩模式与苯环振动的泛音之一之间的费米共振，并且通过将NHCN氘化为NDCN基团可以大大简化。通过红外泵浦-探测实验，确定1中CN伸缩模式在D2O中的振动寿命为0.58±0.04 ps，在H2O中为0.89±0.09 ps，2中在CD3OD/二甲基亚砜中的振动寿命为1.64±0.13 ps，在CH3OH中为0.30±0.05 ps和2.62±0.26 ps。在CH3OH中观察到的2的短时间成分（0.30±0.05 ps）归因于通过费米共振的振动弛豫。这些振动寿命与腈基和叠氮基的相近，但比异腈基的短。因此，氰胺在振动频率和寿命上类似于腈，在跃迁偶极强度上类似于异腈，表现得像是腈和异腈的一种明显的振动杂化体。与弱吸收体腈相比，人们认为氰胺有潜力成为生物分子构象和环境结构及动力学的强吸收红外报告基团。
