SOLVATION OF CYANOALKANES [CH3CN AND (CH3)3CCN] - AN INFRARED AND NUCLEAR MAGNETIC-RESONANCE STUDY
SOLVATION OF CYANOALKANES [CH3CN AND (CH3)3CCN] - AN INFRARED AND NUCLEAR MAGNETIC-RESONANCE STUDY
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DOI：
10.1039/f19888402181
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发表时间：
1988-01-01
期刊：
影响因子：
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通讯作者：
SYMONS, MCR
中科院分区：
文献类型：
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作者：
EATON, G;PENANUNEZ, AS;SYMONS, MCR
研究了CH_3CN和(CH_3)_3CCN稀溶液在一系列非质子性和质子性溶剂中的CN伸缩带(ν_2)。前者引起低频漂移，而后者相对于己烷稀溶液引起高频漂移。在这两种情况下，随着位移的增加，振子强度都有很大的增加。这是第一个具有显示这种二分性的吸收带的溶剂的例子，正常行为是通过非质子化到质子化介质的渐进式低频移动。相比之下，在n.m.r中观察到了这种“正常”行为。~(14)N位移，但~(13)C(CN)位移很小，似乎是随机的。与我们之前对“探针”分子的研究不同，水中溶液的ν2带几乎与甲醇中的一个带相同；然而，在水中的带是单一特征，而在甲醇中是双重的，低频特征靠近未溶剂区。解释是，MeCN在水中完全单溶(氢键)，而在甲醇中只有大约50%单溶。然而，温度变化的影响和对水-非质子混合溶剂体系的研究表明，这可能是不正确的，并且还考虑了MeCN在水中形成两个非常弱的氢键的可能性。甲醇对在低温(-50℃)下被很好地定义，但在变暖时失去了分辨率。在50°C左右，只有一个对称的谱带。对于水-非质子混合溶剂体系，在整个摩尔分数范围内，该带仍然是一个狭窄的单一谱带，没有迹象表明氢键和非氢键单元有双带，这与甲醇在低温下的结果以及我们对其他探针分子的正常经验形成了对比。一种解释是，在氢键和非氢键单元之间存在快速平衡，这在IR上是很快的。时间刻度。本文还报道了其他混合溶剂体系的实验结果。我们尝试在加入MeCN的情况下，利用D2O中HOD的第一和第二泛音O-H伸缩带的变化来测量氢键的数目。这充其量只是定性的，因为对于溶剂化的MeCN来说，O-H带是接近的，而对于水来说，这是‘(OH)自由的’带。然而，对于MeCN，在Me3COH中，(OH)自由和OH-(NCMe)带被清楚地分辨出来。结果支持了MeCN在水中是二水的观点。为了进行比较，对2-氰基-2-甲基丙烷(Me3CCN)进行了不太广泛的研究。结果大致相似，主要区别在于甲醇溶液中氢键的程度降低。
The CN stretching band (ν2) has been studied for dilute solutions of CH3CN and (CH3)3CCN in a range of aprotic and protic solvents. The former induce a low-frequency shift, whereas the latter induce a high-frequency shift relative to dilute solutions in hexane. In both cases there is a large increase in oscillator strength with increasing shift. This is the first example of a solvent that has an absorption band displaying such a dichotomy, the normal behaviour being a progressive low-frequency shift on going via aprotic to protic media. In contrast such ‘normal’ behaviour is observed in the n.m.r. spectra for 14N shifts, but the 13C (CN) shifts are small and seem to be random. In contrast to our previous studies of ‘probe’ molecules the ν2 bands for solutions in water are almost identical to one of the bands in methanol; however, the band in water is a single feature, whilst that in methanol is a doublet, the low-frequency feature being close to the unsolvated region. The interpretation is that MeCN in water is fully monosolvated (hydrogen-bonded), whilst in MeOH it is only ca. 50% monosolvated. However, the effects of temperature changes and studies of mixed water–aprotic solvent systems suggest that this may not be correct, and the possibility that MeCN forms two very weak hydrogen bonds in water is also considered.The methanol doublets are well defined at low temperatures (–50 °C) but resolution is lost on warming. At ca. 50 °C there is only one symmetrical band. For mixed water–aprotic solvent systems, the band remains a narrow singlet throughout the whole mole fraction range, there being no indication of twin bands for hydrogen-bonded and non-hydrogen-bonded units, in contrast with the results for methanol at low temperatures, and our normal experience with other probe molecules. One explanation is that there is rapid equilibrium between hydrogen-bonded and non-hydrogen-bonded units which is fast on the i.r. timescale. Results for other mixed-solvent systems are also reported.We have attempted to use changes in the first and second overtone O—H stretch bands for HOD in D2O on adding MeCN to obtain a measure of the number of hydrogen bonds. This is at best only qualitative because of the proximity of the O—H band for solvated MeCN and the ‘(OH)free’ band for water. However, for MeCN in Me3COH the (OH)free and OH---(NCMe) bands are clearly resolved. The results support the concept that MeCN is dihydrated in water. A less extensive study has been made for 2-cyano-2-methylpropane (Me3CCN) for comparative purposes. The results are broadly similar, the major difference being a reduction in the extent of hydrogen bonding in methanolic solutions.