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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(金幣+50, 翻譯EPI+1): 2010-05-11 19:01:12
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4 結(jié)果與分析 4.1 分子幾何構(gòu)型 以6-31G（d.p）為基組水平，用 ab initio HF 和 DFT-B3LYP的方法計算出 p-對甲氧基苯甲醛的最佳結(jié)構(gòu)參數(shù)后列在表1中并與圖3的原子編碼表相對應(yīng)。把表1中計算出的p-對甲氧基苯甲醛的鍵長和鍵角與從試驗中用x-射線衍射法得到的有效數(shù)據(jù)【10】進(jìn)行比較，我們發(fā)現(xiàn)理論值上大多數(shù)的最佳鍵角值比試驗值要略微大些，而且，認(rèn)為這是計算條件不同造成的，即理論上的計算是針對氣相中隔離的分子進(jìn)行的，而實驗值的結(jié)果是對固態(tài)中的分子進(jìn)行的。當(dāng)把用B3LYP和HF的方法對鍵長和鍵角計算的值進(jìn)行比較時，前者在總體水平上高于后者，但后者更接近實驗的結(jié)果。雖然結(jié)果不同，但能體現(xiàn)幾何參數(shù)的近似性，可以做為計算其他參數(shù)，比如振動頻率和熱力學(xué)性質(zhì)的基礎(chǔ)。 4.2 振動的分類 人們認(rèn)為.3.4-二甲基苯甲醛的幾何構(gòu)型為除了甲基基團中的氫原子外，其他原子都在相同的平面上。Sundaraganesan 等人【17】認(rèn)為此化合物就是一個平面。從對力場的計算中，Zwarich 等人【18】認(rèn)為苯甲醛分子是一個平面。就目前研究而言，甲基基團中的氫和碳被認(rèn)為處在同一個平面上。用這個假設(shè)的結(jié)構(gòu)模型，正在研究的分子屬于銫基團和覆蓋48個常規(guī)振動模型 21A+17A。使用3-31G（d.p） 為基組水平，用HF和B3LYP法計算的p-對甲氧基苯甲醛的諧波振動頻率連同漫射和極化函數(shù)列在表2和表3中。 若忽略實際情況中波的不和諧情況，經(jīng)比較發(fā)現(xiàn)用HF和B3LYP法計算的頻率值（表4）總體上高于實驗所得值（密度上電子對應(yīng)的funk-理論在一定程度上使得頻率值小于HF法所得的值）。減少的質(zhì)量和力的常數(shù)以及所計算的頻率的去極化率同時列在表2和表3中，而且，也是分類的基礎(chǔ)。用6-3G(d.p)為基組水平，對p-對甲氧基苯甲醛用HF和B3LYP法所得的窄光譜圖用圖4表示。 在每一個基波峰值內(nèi)，所有的計算模式按頻率由大到小被編號。把在各種振動的模式下觀測到FTIR和FT Raman的頻率進(jìn)行分類后列在表4. |
7樓2010-05-10 17:12:27
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