Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, USA.
J Am Chem Soc. 2011 Mar 2;133(8):2632-41. doi: 10.1021/ja109218j. Epub 2011 Feb 4.
Single-conformation spectroscopy of the three lignin monomers (hereafter "monolignols") p-coumaryl alcohol (pCoumA), coniferyl alcohol (ConA), and sinapyl alcohol (SinA) has been carried out on the isolated molecules cooled in a supersonic expansion. Laser-induced fluorescence excitation, dispersed fluorescence, resonant two-photon ionization, UV-UV hole-burning, and resonant ion-dip infrared spectroscopy were carried out as needed to obtain firm assignments for the observed conformers of the three molecules. In each case, two conformers were observed, differing in the relative orientations of the vinyl and OH substituents para to one another on the phenyl ring. In pCoumA, the two conformers have S(0)-S(1) origins nearly identical in size, split from one another by only 7 cm(-1), in close analogy with previous results of Morgan et al. on p-vinylphenol ( Chem. Phys. 2008 , 347 , 340 ). ConA, with its methoxy group ortho to the OH group, also has two low-energy conformers forming a syn/anti pair, in this case with the OH group locked into an orientation in which it forms an intramolecular H-bond with the adjacent methoxy group. The electronic frequency shift between the two conformers is dramatically increased to 805 cm(-1), with the dominant conformer of ConA (with S(0)-S(1) origin at 32 640 cm(-1)) about 5 times the intensity of its minor counterpart (with S(0)-S(1) origin at 33 444 cm(-1)). The presence of an OH···OCH(3) intramolecular H-bond is established by the shift of the OH stretch fundamental of the OH group to 3599 cm(-1), as it is in o-methoxyphenol ( Fujimaki et al. J. Chem. Phys. 1999 , 110 , 4238 ). Analogous single-conformation UV and IR spectra of o-methoxy-p-vinylphenol show a close similarity to ConA and provide a basis for a firm assignment of the red-shifted (blue-shifted) conformer of both molecules to the syn (anti) conformer. The two observed conformers of SinA, with its two methoxy group straddling the OH group, have S(0)-S(1) origins split by 239 cm(-1) (33 055 and 33 294 cm(-1)), a value between those in pCoumA and ConA. A combination of experimental data and calculations on the three monolignols and simpler derivatives is used to establish that the conformational preferences of the monolignols reflect the preferences of each of the ring substituents separately, enhanced by the presence of the intramolecular OH···OCH(3) H-bond. Taken as a whole, the presence of multiple flexible substituents locks in certain preferred orientations of the groups relative to one another, even in the apparently flexible allyl alcohol side chain (-CH═CH-CH(2)OH), where the OH group orients itself so that the hydrogen is pointed back over the vinyl π cloud in order to minimize interactions between the oxygen lone pairs and the π electrons.
已在超音速膨胀中冷却的三种木质素单体（以下简称“木质素单体”）松柏醇（松柏醇）、丙烯醇（丙烯醇）和丁香醇（丁香醇）的单构象光谱进行了研究。激光诱导荧光激发、分散荧光、共振双光子电离、紫外-紫外孔烧蚀和共振离子陷红外光谱在需要时进行，以确定三种分子观察到的构象的确切归属。在每种情况下，都观察到两种构象，它们在苯环上彼此邻位的乙烯基和 OH 取代基的相对取向不同。在 pCoumA 中，两个构象的 S(0)-S(1)起源大小几乎相同，仅相差 7 cm(-1)，与 Morgan 等人之前对 p-乙烯基苯酚的结果非常相似。酚（化学物理。2008 年，347 卷，340 页）。由于其甲氧基位于 OH 基团的邻位，ConA 也具有两个低能量构象，形成一个顺/反对，在这种情况下，OH 基团被锁定在与相邻甲氧基形成分子内氢键的取向中。两个构象之间的电子频率位移显著增加到 805 cm(-1)，ConA 的主要构象（S(0)-S(1)起源于 32 640 cm(-1)）的强度约为其次要构象（S(0)-S(1)起源于 33 444 cm(-1)）的 5 倍。OH 基团的伸缩基本振动向 3599 cm(-1)的移动证实了 OH···OCH(3)分子内氢键的存在，就像在邻甲氧基苯酚中一样（ Fujimaki 等人。化学物理。1999 年，110 卷，4238 页）。o-甲氧基-p-乙烯基苯酚的类似单构象 UV 和 IR 光谱与 ConA 非常相似，并为两种分子的红移（蓝移）构象的明确归属提供了基础。丁香醇的两个观察到的构象，其两个甲氧基跨越 OH 基团，S(0)-S(1)起源分裂 239 cm(-1)（33 055 和 33 294 cm(-1)），介于 pCoumA 和 ConA 之间。对三种木质素单体和更简单衍生物的实验数据和计算的组合用于确定木质素单体的构象偏好反映了每个环取代基各自的偏好，同时增强了分子内 OH···OCH(3)氢键的存在。总的来说，多个灵活取代基的存在将彼此之间的某些基团锁定在某些优选取向中，即使在明显灵活的丙烯醇侧链（-CH═CH-CH(2)OH）中也是如此，其中 OH 基团定向使氢指向乙烯基π云的背面，以最小化氧孤对和π电子之间的相互作用。
