# Electric Field and Temperature Effects on the Ab Initio Spectroscopy of Liquid Methanol
## 3 Results and Discussion
Moreover, such a phenomenon, known as the vibrational Stark effect [44; 45], is progressive with respect to the increment of the field strength, as it is visible in the spectra emerging from AIMD simulations carried out at several field intensities and displayed in Figure 2. This field-induced effect is fully stackable with that observed in liquid water upon exposure to static external electric fields [47], indicating that the collective vibrational response of polar liquids to externally applied electrostatic potential gradients exhibits some common features. However, whereas in bulk water the peak's height and, to some extent, also the full-width at half-maximum (FWHM) of the OH-stretching band are not altered by the field [47], in liquid methanol they change significantly upon field exposure. In fact, the height of the peak of the OH-stretching band progressively decreases with the increase in the field intensity, as shown in Figure 2. More complicated is the interpretation of the effects of the field on the FWHM of the OH-stretching band because of the combined effect carried by the red-shift imposed by the field and the presence of the closely-located--at lower frequencies--CH\({}_{3}\)-stretching band. The field-induced red-shift of the OH-stretching band leads, indeed, to a partial mixing of the typical vibrational frequencies of this mode with those of the CH\({}_{3}\) stretching. In this way, the application of intense electric fields (i.e., \(E\geq 0.25\) V/A) disrupts the separation of those bands in the IR spectrum of liquid methanol, as shown by the blue and violet curves in Figure 2 at frequencies around \(\sim\)3000 cm\({}^{-1}\). Although the CH\({}_{3}\)-bending mode--whose frequencies of vibration fall at \(\sim\)1400 cm\({}^{-1}\)--appears to be less affected by the application of the field, a slight but visible blue-shift accompanied by an intensity reduction is recorded at the highest field intensities here explored, as shown in Figure 2. Whereas no appreciable field-induced frequency shifts are exhibited by the CH\({}_{3}\)-rocking band, the height of the respective peak decreases as the intensity of the field is increased, as shown in Figure 2. At lower frequencies, although a slight red-shift is observed, the very narrow CO-stretching band is only weakly altered by the field up to strengths equal to 0.25 V/A, a threshold marking an abrupt decrease  
Figure 2: IR absorption spectra of bulk liquid methanol determined in absence (red line) and under different field intensities (colored from orange to violet) as detailed in the legend at 300 K.  
in the intensity of such a band. At the same time, the out-of-plane OH-bending band is progressively blue-shifted as the field strength is increased and the height of its peak increases as well from \(E=0.25\) V/A. Thus, the application of static electric fields sensibly modifies the vibrational features of bulk liquid methanol. Furthermore, starting from field intensities equal to 0.25 V/A, an abrupt change of several IR bands is recorded, indicating the onset of structurally important modifications at the molecular level. Methanol IR spectra for different temperatures are shown in Figure 3. Despite a physiologic increase of the intensity of some bands accompanied by a slight narrowing of the allowed frequencies of vibration (i.e., the OH-stretching, the CH\({}_{3}\)-stretching, the CH\({}_{3}\)-bending, and the out-of-plane OH-bending bands), no systematic shift in the average frequency of vibration is recorded upon decreasing the external temperature, with only one exception; in fact, the frequencies of the out-of-plane OH-bending mode of vibration undergo a slight increase upon decreasing the temperature, as shown in the low-frequency region of Figure 3, a trend which coincides with that induced by an increment of the strength of an external electric field.