Resonant two-photon ionization (R2PI), resonant ion-dip infrared (RIDIR), and UV-UV hole-burning spectroscopies have been employed to obtain conformation-specific infrared and ultraviolet spectra under supersonic expansion conditions for O-(2-acetamidoethyl)-N-acetyltyramine (OANAT), a doubly substituted aromatic in which amide-containing alkyl and alkoxy side chains are located in para positions on a phenyl ring. For comparison, three single-chain analogs were also studied: (i) N-phenethyl-acetamide (NPEA), (ii) N-(p-methoxyphenethyl-acetamide) (NMPEA), and (iii) N-(2-phenoxyethyl)-acetamide (NPOEA). Six conformations of OANAT have been resolved, with S(0)-S(1) origins ranging from 34,536 to 35,711 cm(-1), denoted A-F, respectively. RIDIR spectra show that conformers A-C each possess an intense, broadened amide NH stretch fundamental shifted below 3400 cm(-1), indicative of the presence of an interchain H bond, while conformers D-F have both amide NH stretch fundamentals in the 3480-3495 cm(-1) region, consistent with independent-chain structures with two free NH groups. NPEA has a single conformer with S(0)-S(1) origin at 37,618 cm(-1). NMPEA has three conformers, two that dominate the R2P1 spectrum, with origin transitions between 35,580 and 35,632 cm(-1). Four conformations, one dominate and three minor, of NPOEA have been resolved with origins between 35,654 and 36,423 cm(-1). To aid the making of conformational assignments, the geometries of low-lying structures of all four molecules have been optimized and the associated harmonic vibrational frequencies calculated using density functional theory (DFT) and RIMP2 methods. The S(0)-S(1) adiabatic excitation energies have been calculated using the RICC2 method and vertical excitation energies using single-point time-dependent DFT. The sensitivity of the S(0)-S(1) energy separation in OANAT and NPOEA primarily arises from different orientations of the chain attached to the phenoxy group. Using the results of the single-chain analogs, tentative assignments have been made for the observed conformers of OANAT. The RIMP2 calculations predict that interchain H-bonded conformers of OANAT are 25-30 kJ/mol more stable than the extended-chain structures. However, the free energies of the interchain H-bonded and extended structures calculated at the preexpansion temperature (450 K) differ by less than 10 kJ/mol, and the number of extended structures far outweighs the number of H-bonded conformers. This entropy-driven effect explains the presence of the independent-chain conformers in the expansion, and cautions future studies that rely solely on relative energies of conformers in considering possible assignments.

中文翻译：

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具有两个柔性侧链的原型分子中的熵驱动种群分布：O-（2-乙酰氨基乙基）-N-乙酰酪胺。

共振双光子电离（R2PI），共振离子浸入红外（RIDIR）和UV-UV空穴燃烧光谱法已被用于在超声膨胀条件下获得O-（2-乙酰氨基乙基）的特定构象的红外和紫外光谱-N-乙酰基酪胺（OANAT），一种双取代的芳族化合物，其中含酰胺的烷基和烷氧基侧链位于苯环的对位。为了比较，还研究了三种单链类似物：（i）N-苯乙基乙酰胺（NPEA），（ii）N-（对甲氧基苯乙基乙酰胺）（NMPEA）和（iii）N-（2-苯氧基乙基） ）-乙酰胺（NPOEA）。解析了OANAT的六个构象，S（0）-S（1）的起源分别从34,536到35,711 cm（-1），分别表示为AF。RIDIR光谱表明，构象AC各自具有强烈的 加宽的酰胺NH拉伸基本面移动到3400 cm（-1）以下，表明存在链间H键，而构象异构体DF的两个酰胺NH拉伸基本面都在3480-3495 cm（-1）区域，与独立链一致具有两个游离NH基的结构。NPEA具有一个单一构象体，起源于37,618 cm（-1）的S（0）-S（1）。NMPEA具有三个构象异构体，其中两个在R2P1光谱中占主导地位，其原点跃迁在35,580和35,632 cm（-1）之间。NPOEA的四个构型，一个占优势，三个次要，已被解析，起源于35,654至36,423 cm（-1）之间。为了帮助进行构象分配，已对所有四个分子的低层结构的几何结构进行了优化，并使用密度泛函理论（DFT）和RIMP2方法计算了相关的谐波振动频率。已使用RICC2方法计算了S（0）-S（1）绝热激发能，并使用了单点时变DFT计算了垂直激发能。OANAT和NPOEA中S（0）-S（1）能量分离的敏感性主要来自与苯氧基相连的链的不同方向。利用单链类似物的结果，对观察到的OANAT构象异构体进行了初步分配。RIMP2计算预测，OANAT的链间氢键构象异构体比扩展链结构稳定25-30 kJ / mol。然而，在预膨胀温度（450 K）下计算出的链间氢键键合结构和扩展结构的自由能相差不到10 kJ / mol，并且扩展键结构的数量远远超过氢键键合构象体的数量。