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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Ab Initio Study of Proton Chemical Shift in Supercritical Methanol Using Gas-Phase Approximation
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Ab Initio Study of Proton Chemical Shift in Supercritical Methanol Using Gas-Phase Approximation

机译：气相近似从头算研究超临界甲醇中质子的化学位移

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摘要

Chemical shifts of the OH proton in supercritical methanol referenced to the methyl proton of methanol monomer have been estimated theoretically using the ab initio molecular orbital (MO) method. The degree of dissociation from hydrogen-bonded methanol clusters to monomers calculated using the CCSD(T)/6-31+G-(d)//MP2(frozen-core)/6-31+G(d) level of theory indicates that supercritical methanol is comprised of 89% monomer and 10% cyclic tetramer plus ～1% dimer at the critical point (T_c = 512.6 K; P_c = 8.09 MPa). The predominant existence of the cyclic tetramers rather than the dimers in supercritical methanol is in contrast to previous theoretical results for supercritical water that indicate the composition of, except for 80% monomer, 20% dimer with little existence of a larger size of clusters at the critical point (T_c = 647.1 K; P_c = 22.06 MPa). It is also found that a significant fluctuation of the composition of methanol should be caused by a greater change in the degree of dissociation of the cyclic tetramer near the critical point. On the basis of the above supercritical methanol composition, the chemical shift of the OH proton is determined to be -2.00 ppm at the MP2(frozen-core)/6-31+G(d)//MP2(frozen-core)/6-31+G(d) level of theory, which excellently reproduces the recent NMR experimental results.
机译：理论上已经使用从头算分子轨道（MO）方法估算了超临界甲醇中的OH质子相对于甲醇单体的甲基质子的化学位移。使用CCSD（T）/ 6-31 + G-（d）// MP2（冷冻核）/ 6-31 + G（d）理论计算得出的氢键合甲醇团簇与单体的解离度表明在临界点（T_c = 512.6 K; P_c = 8.09 MPa），超临界甲醇由89％单体和10％环状四聚体加〜1％二聚体组成。与超临界水中先前的理论结果相反，超临界甲醇中主要存在环状四聚体而不是二聚体，该理论结果表明，除80％的单体外，20％的二聚体的组成几乎不存在，而在超临界甲醇中几乎不存在较大尺寸的簇。临界点（T_c = 647.1 K; P_c = 22.06 MPa）。还发现，应该由接近临界点的环状四聚体的解离度的较大变化引起甲醇组成的显着波动。基于以上超临界甲醇组成，在MP2（冷冻核）/ 6-31 + G（d）// MP2（冷冻核）/下，OH质子的化学位移确定为-2.00ppm。 6-31 + G（d）的理论水平，可以很好地再现最近的NMR实验结果。

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