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4. Results and discussion 4.1. Molecular geometry The optimized structure parameters of p-anisaldehyde calcu- lated by ab initio HF and DFT-B3LYP levels with the 6-31G(d,p) basis set are listed in the Table 1 in accordance with the atom numbering scheme given in Fig. 3. Table 1 compares the cal- culated bond lengths and angles for p-anisaldehyde with those experimentally available from X-ray diffraction data [10]. From the theoretical values, we can find that most of the optimized bond angles are slightly larger than the experimen- tal values, due to the theoretical calculations belong to isolated molecules in gaseous phase and the experimental results belong to molecules in solid state. Comparing bond angles and lengths of B3LYP with those of HF, as a whole the formers are on higher side than the latter and the HF calculated values cor- relates well compared with the experimental results. In spite of the differences, calculated geometric parameters represent a good approximation and they are the bases for calculating other parameters, such as vibrational frequencies and thermodynamic properties. 4.2. Vibrational assignments It has been argued that in 3,4-dimethylbenzaldehyde, all the atoms excepting the H atoms of CH3group are in the same plane. Sundaraganesan et al. [17] took the mentioned compound to be planar. During the force field calculations, Zwarich et al. [18] took the benzaldehyde molecule to be planar. Therefore in the present study, C and H atoms of CH3group are taken to be in plane of the methyl group. With this assumed struc- tural model, the molecule under consideration would belongs to Cspoint group and the 48 normal modes of vibration which span the irreducible representations: 21 A + 17 A . The har- monic vibrational frequencies calculated for p-anisaldehyde at HF and B3LYP levels using the 6-31G(d,p) basis set along with diffuse and polarization functions have been summarized in Tables 2 and 3. Comparison of the frequencies calculated at HF and B3LYP with experimental values (Table 4) reveals the overestimation of the calculated vibrational modes due to neglect of anharmonicity in real system. Inclusion of electron correlationtional in density func-theory to a certain extend makes the frequency values smaller in comparison with the HF frequency data. The reduced mass and force constants along with the depolarization ratios of the calculated frequencies have been included in Tables 2 and 3 to have rational basis for the assignments. The stick spectra of p-anisaldehyde at HF and B3LYP levels using 6-31G(d,p) basis set have been shown in Fig. 4. . All the calculated modes are numbered from the largest to the smallest frequency within each fundamental wave number. The observed FTIR and FT Raman frequencies for various modes of vibrations are assigned and are presented in the Table 4 |
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huoyan520(金币+50, 翻译EPI+1): 2010-05-11 19:01:12
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4 结果与分析 4.1 分子几何构型 以6-31G（d.p）为基组水平，用 ab initio HF 和 DFT-B3LYP的方法计算出 p-对甲氧基苯甲醛的最佳结构参数后列在表1中并与图3的原子编码表相对应。把表1中计算出的p-对甲氧基苯甲醛的键长和键角与从试验中用x-射线衍射法得到的有效数据【10】进行比较，我们发现理论值上大多数的最佳键角值比试验值要略微大些，而且，认为这是计算条件不同造成的，即理论上的计算是针对气相中隔离的分子进行的，而实验值的结果是对固态中的分子进行的。当把用B3LYP和HF的方法对键长和键角计算的值进行比较时，前者在总体水平上高于后者，但后者更接近实验的结果。虽然结果不同，但能体现几何参数的近似性，可以做为计算其他参数，比如振动频率和热力学性质的基础。 4.2 振动的分类 人们认为.3.4-二甲基苯甲醛的几何构型为除了甲基基团中的氢原子外，其他原子都在相同的平面上。Sundaraganesan 等人【17】认为此化合物就是一个平面。从对力场的计算中，Zwarich 等人【18】认为苯甲醛分子是一个平面。就目前研究而言，甲基基团中的氢和碳被认为处在同一个平面上。用这个假设的结构模型，正在研究的分子属于铯基团和覆盖48个常规振动模型 21A+17A。使用3-31G（d.p） 为基组水平，用HF和B3LYP法计算的p-对甲氧基苯甲醛的谐波振动频率连同漫射和极化函数列在表2和表3中。 若忽略实际情况中波的不和谐情况，经比较发现用HF和B3LYP法计算的频率值（表4）总体上高于实验所得值（密度上电子对应的funk-理论在一定程度上使得频率值小于HF法所得的值）。减少的质量和力的常数以及所计算的频率的去极化率同时列在表2和表3中，而且，也是分类的基础。用6-3G(d.p)为基组水平，对p-对甲氧基苯甲醛用HF和B3LYP法所得的窄光谱图用图4表示。 在每一个基波峰值内，所有的计算模式按频率由大到小被编号。把在各种振动的模式下观测到FTIR和FT Raman的频率进行分类后列在表4. |
7楼2010-05-10 17:12:27