Xu Ben, Hirsch Anna, Kronik Leeor, Poduska Kristin M
Department of Physics and Physical Oceanography, Memorial University of Newfoundland St. John's Canada
Department of Materials and Interfaces, Weizmann Institute of Science Rehovoth Israel.
RSC Adv. 2018 Oct 3;8(59):33985-33992. doi: 10.1039/c8ra06608f. eCollection 2018 Sep 28.
Isotope enrichment is widely used to affect atomic masses, facilitating data acquisition and peak assignments in experiments such as nuclear magnetic resonance and infrared spectroscopy. It is also used for elucidating the origin of weak features in systems where natural isotopic abundances are low. However, it is not possible to always know precisely how vibrational modes change for arbitrary levels of isotopic substitution. Here, we examine this issue by presenting a joint experimental and theoretical study for the important case of C isotope substitution effects on the infrared spectra of calcite. By systematically varying the C : C ratio, we find that the relative positions and intensities of infrared-active vibrational modes can vary, in a non-linear and mode-dependent fashion, with minority isotope content and proximity. This allows us to determine the origin of weak spectral features due to the natural abundance of isotopes and to show that even relatively low levels of substitution are not necessarily within the "dilute limit," below which isotopic substitutions do not interact.
同位素富集被广泛用于影响原子质量，在核磁共振和红外光谱等实验中有助于数据采集和峰归属。它还用于阐明自然同位素丰度较低的体系中微弱特征的起源。然而，对于任意水平的同位素取代，不可能总是精确地知道振动模式如何变化。在这里，我们通过对方解石红外光谱中碳同位素取代效应这一重要情况进行联合实验和理论研究来探讨这个问题。通过系统地改变碳与碳的比例，我们发现红外活性振动模式的相对位置和强度会以非线性且依赖于模式的方式随少数同位素含量和邻近度而变化。这使我们能够确定由于同位素自然丰度导致的微弱光谱特征的起源，并表明即使相对较低水平的取代也不一定处于“稀释极限”内，低于该极限同位素取代不会相互作用。
