2014年6月英语四级作文真题附范文(第2套):A Beautiful City in China

句法很棒，若适当增加一些从句的使用，文章会取得更好的成绩；文中词汇表达灵活性要提高，希望能较熟练地使用高级词汇；可适当增加连接词和衔接词的使用。

Fig. 1 gives the IR spectra in the region from 4000 to 400 cm–1. The bands around 3430 and 1628 cm–1 observed in all samples are assigned to the symmetrical stretching vibrations of –OH groups on the surface of Fe3O4 as well as MCM-41 and the bending vibration of adsorbed water [22,25], respectively. The asymmetric stretching vibrations and symmetric stretching vibrations of Si-O-Si bands are situated at 1085 and 803 cm–1, respectively [27]. In addition, the band at 552 cm–1 belongs to the vibration of Fe-O bond [29]. In the spectrum of Cu(acac)2/n-Bu4NBr/Fe3O4@MCM-41 (Fig. 1c), bands at 2963 and 2927 cm−1 are assigned to the asymmetric vibrations of –CH3 and –CH2 units, respectively. 2866 and 1384 cm−1 are ascribed to the symmetric stretching and bending vibrations of –CH3 units, respectively. The bands at 1565 and 613 cm–1 correspond to the symmetrical stretching vibrations of C=C bond and the stretching vibration of Cu-O bond, respectively [30]. In addition, the C-N stretching vibration also occurs at 1230 cm−1. The appearance of these characteristic bands related to Cu(acac)2 and n-Bu4NBr in the spectrum of Cu(acac)2/n-Bu4NBr/Fe3O4@MCM-41 (Fig. 1c) reveals the introduction of active species including Cu(acac)2 and n-Bu4NBr.