共聚焦拉曼光谱研究过饱和硝酸铵液滴中NH4+, NO3- 和H2O之间的相互作用

将硝酸铵液滴沉积在石英基底上,通过降低该液滴周围环境的相对湿度,测定了该液滴由低浓度直至过饱和状态下高信噪比的拉曼光谱.其中,相对湿度的变化可以精确控制液滴浓度的改变.在相对湿度(RH)由72.1%降低至37.9%的过程中,硝酸铵液滴v1-NO-3峰位保持在1048cm-1,半峰宽为10cm-1.该现象表明NO-3周围的水分子被NH4+取代后不会对v1-NO-3造成影响,说明水分子和NH4+所形成的氢键具有相同的强度.对2500-4000cm-1范围内的拉曼光谱进行成分分析,2890、3090、3140、3220、3402及3507cm-1分别被指认为NH+4伞状弯曲振动的泛频、NH+4伞状弯曲振动与摇摆振动的组合谱带、NH+4的对称伸缩振动、NH+4的反对称伸缩振动、水峰中强氢键成分和弱氢键成分.从拟合结果得出:强氢键在氢键结构中所占百分含量随液滴相对湿度的降低而减少,弱氢键所占百分含量随液滴相对湿度的降低而增加.该变化趋势是NO-3和NH+4之间复杂相互作用的结果.

Confocal Raman Spectroscopy Studies on the Interactions between NH4+,NO3- and H2O in Supersaturated NH4NO3 Droplets

High signal to noise (S/N) ratio Raman spectra of NH4NO3 droplets deposited on a quartz substrate were obtained from dilute to supersaturated states by reducing the relative humidity (RH) of the environment, allowing for accurate control over the concentration of solute within the droplet. When the RH was reduced from 72.1% to 37.9%, the peak position of the v1-NO3- band of the NH4NO3 droplet did not shift from its original position at 1048 cm-1 and a similar full width at half-maximum (FWHM) of 10 cm-1 was also observed. It was concluded that the replacement of H2O molecules hydrogen-bonded with the O atoms of NO-3 with NH4+ ions leaves the frequency of v1-NO3- relatively unchanged, indicating that both H2O and NH4+ forming hydrogen bonds have the same strength. From component band analysis in the spectral range of 2500-4000 cm-1, six peaks at 2890, 3090, 3140, 3220, 3402, 3507 cm-1 were identified and assigned. The first four components were assigned to the second overtone of NH4+ umbrella bending, the combination band of NH4+ umbrella bending and rocking vibrations, the NH4+ symmetric stretching vibration, and the NH4+ antisymmetric stretching vibration. The latter two peaks originated from strong and weak hydrogen bonds. The signature of the strong hydrogen bonding component was observed to decrease in intensity with the decrease in RH over the full range from 72.1% to 37.9%, while the signature of the weak hydrogen bonding component was shown to increase as the RH was reduced. The observed trend in the hydrogen bonding component resulted from the interactions between NH+4 and NO-3.