Urquhart Stephen G, Gillies Rob
Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, SK S7N 5C9, Canada.
J Chem Phys. 2006 Jun 21;124(23):234704. doi: 10.1063/1.2206589.
The carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectra of simple gaseous alkane molecules differ from the spectra of the same alkane molecules in the condensed phase. The origin of these large, systematic differences is poorly understood. The NEXAFS spectra of gaseous alkanes are interpreted as a progression of core-->Rydberg transitions with distinctive vibronic structure. The interpretation of the NEXAFS spectra of condensed phase alkanes is varied. Specifically, the degree of Rydberg character in the pre-edge core excited states of condensed alkanes is controversial. We determined the character of core excited states in condensed alkanes with a combination of experiment and computational study. From this, we have determined the nature of matrix effects for these species. The high-resolution carbon 1s NEXAFS spectrum of gaseous neopentane is dramatically different from its condensed phase spectrum, a striking illustration of the dramatic spectroscopic changes that occur upon condensation. High quality ab initio calculations of a cluster designed to model the solid phase environment provide definitive evidence for the reduction of Rydberg character and support the assignment of sigma*C-H) valence character in the pre-edge features in the NEXAFS spectra of condensed alkanes.
简单气态烷烃分子的碳1s近边X射线吸收精细结构（NEXAFS）光谱与处于凝聚相的相同烷烃分子的光谱不同。这些巨大的、系统性差异的起源尚不清楚。气态烷烃的NEXAFS光谱被解释为具有独特振动电子结构的芯-里德堡跃迁的进展。凝聚相烷烃的NEXAFS光谱的解释多种多样。具体而言，凝聚态烷烃前边缘芯激发态中里德堡特征的程度存在争议。我们通过实验和计算研究相结合的方法确定了凝聚态烷烃中芯激发态的特征。据此，我们确定了这些物种的基体效应的性质。气态新戊烷的高分辨率碳1s NEXAFS光谱与其凝聚相光谱显著不同，这是凝聚时发生的显著光谱变化的一个突出例证。对旨在模拟固相环境的团簇进行的高质量从头算计算为里德堡特征的降低提供了确凿证据，并支持在凝聚态烷烃的NEXAFS光谱的前边缘特征中σ*（C-H）价特征的归属。
