Ab initio calculation of amide carbonyl stretch vibrational frequencies in solution with modified basis sets.: 1.: N-methyl acetamide
Ab initio calculation of amide carbonyl stretch vibrational frequencies in solution with modified basis sets.: 1.: N-methyl acetamide
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DOI：
10.1021/jp013203y
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发表时间：
2001-12-06
影响因子：
2.9
通讯作者：
Keiderling, TA
中科院分区：
文献类型：
--
作者：
Kubelka, J;Keiderling, TA
在密度泛函理论（DFT）BPW 91水平上，采用改进的6- 31 G（d）基组，对小分子酰胺N-甲基乙酰胺（NMA）进行了计算.作为计算酰胺I和酰胺II频率的一种有效方法，可直接与通常在溶液中测量的频率进行比较，计算结果与实验测量的气相和溶液相的FTIR光谱进行了比较。与在HF、CASSCF、MP2、QCISD和CCD水平上用相同基组计算的酰胺I和II频率相比，在DFT水平上设置的6- 31 G（d）基产生的振动频率与气相实验具有最佳的一致性。DFT（BPW 91）/6- 31 G（d）能级计算。3 H（2）O氢键复合物与Onsager或CPCM反应场产生酰胺I，II和III频率与水溶液中的实验相当。发现酰胺I频率和较小程度上的酰胺II频率对基集中d函数的指数敏感。在6- 31 G（d）基组中使用更扩散（更小指数）的d函数导致计算的酰胺I频率更接近溶液实验值。如此修饰。相对小的基组可以提供近似溶剂对酰胺振动频率的影响的计算上有效的方法。
Density functional theory DFT(BPW91) level calculations with modified 6-31G(d) basis sets are tested for a small amide, N-methyl acetamide (NMA). as an efficient way for calculating amide I and amide II frequencies that are directly comparable to those commonly measured in solution, The calculational results are compared to experimentally measured FTIR spectra in gas and solution phases. The 6-31G(d) basis set at the DFT level yields vibrational frequencies that have the best agreement with the gas-phase experiment, as compared to amide I and II frequencies calculated with the same basis at the HF, CASSCF, MP2, QCISD, and CCD levels. The DFT(BPW91)/6-31G(d) level calculation for the NMA . 3H(2)O hydrogen-bonded complex with an Onsager or CPCM reaction field yields amide I, II, and III frequencies comparable to the experiment in aqueous solution. The amide I and, to a smaller degree, amide II frequencies are found to be sensitive to the exponent of the d function in the basis set. Use of more diffuse (smaller exponent) d functions in the 6-31G(d) basis set results in a calculated amide I frequency closer to the solution experimental values. Such modified. relatively small basis sets may provide a computationally efficient means of approximating the solvent effects on amide vibrational frequencies.