量子化学计算辅助聚(N-异丙基丙烯酰胺)溶液碳谱的归属

^(13)C HIGH-RESOLUTION SPECTRUM OF POLY(N-ISOPROPYLACRYLAMIDE) AS ASSIGNED WITH THE AID OF GAUSSIAN98 QUANTUM CHEMICAL CALCULATION

测定了聚 (N 异丙基丙烯酰胺 ) (PNIPAAM)的溶液高分辨定量1 3CNMR谱 ,使用γ gauche效应和Gaussian98从头计算GIAO CHF两种方法分别计算了各立构序列的化学位移 .结果表明在计算结果与实验数据之间的符合程度方面 ,从头计算方法明显优于γ gauche效应方法 .利用从头计算的结果对谱图作了相应归属 ,并分析了PNIPAAM的立构规整度 .结果表明PNIPAAM链的增长服从Bernoulli过程 ,立构序列满足无规分布 .这说明量化计算和NMR相结合是研究高分子高级链结构的有效方法 .

The quantitative 13 C high resolution NMR spectrum of poly(N-isopropylacrylamide) (PNIPAAM) was assigned with the aid of Gaussian98 calculation. Two methods, γ-gauche effect and ab initio calculation, have been applied to calculate the 13 C NMR chemical shifts of stereosequences of PNIPAAM. The parameters for γ-gauche effect were developed from the ab initio calculations of diads. The basis set used was 6-31G+*, and the method was GIAO-CHF. For ab initio calculation, all geometries were optimized via AM1 method and the 13 C NMR chemical shifts were calculated via GIAO-CHF method with STO-3G basis set. Stereosequences to triad level were considered for the methine carbon of main chain, while for the methylene and methyl carbons, stereosequences to tetrad and pentad level were consideredrespectively. It was found that the ab initio calculation can well reproduce the observed 13 C NMR spectrum, while the γ-gauche effect method failed to give satisfactory result. Thus, based on the result of ab initio calculation, the signals of methine carbons, methylene carbon and methyl carbon were assigned and the tacticity of PNIPAAM was turn out to be random. As a result, the ab initio calculation was proved to be an effective method to investigate the tacticities of macromolecules. ;