# Electric Field and Temperature Effects on the Ab Initio Spectroscopy of Liquid Methanol
## 3 Results and Discussion
However, differently from what is observed in liquid water, where the application of a static electric field leads to a more \"ice-like\" structured liquid [47], in neat liquid methanol the effects induced by the field are definitely separated from those induced by a decrease in the temperature. Generally speaking, the features induced by the application of a vector (i.e., the oriented electric field) and those afforded by varying a scalar quantity (i.e., the temperature) on matter are by their own nature a priori deeply different. Thus, the higher multiplicity of the H-bond network present in liquid water--where each molecule is on average involved in \(\sim\)4 H-bonds--with respect to that of methanol--where each molecule is on average involved in only \(\sim\)2 H-bonds--significantly mitigates the differences carried by an electric field increase and a temperature decrease. In fact, whereas in water the direction imposed by the external field to the dipole vectors can lead to H-bonded solid-like structures, in methanol it can only lead to linear chains of H-bonded molecules all oriented along the field axis and with the hydrophilic heads placed in favor of the field [49; 50], a circumstance preventing the formation of any methanol solid phase [79]. This way, whilst electrofreezing may in principle be possible in water, as suggested in the literature [46; 47; 48], it appears to be unlikely in methanol. In fact, as also shown in Table 1, the seminal effect produced by the external field is that of decreasing the average number of H-bonds <\(n_{HB}\)>--as determined from geometric criteria [20; 23]--in which a methanol molecule is statistically involved. Nevertheless, once a fraction of molecules has been re-oriented by the field [49], a slight increase in the number of H-bonds is recorded, even though it is up to values significantly smaller than those observed in absence of the field, as reported in Figure 1.0 Beyond 0.30 V/A, some molecular dissociations occur, leading to correlated proton-transfer events [49]. Although proton transfer takes place across the H-bond network, the average lifetime of H-bonds in liquid methanol decreases for field intensities higher than those explored in the current investigation (i.e., above 0.30 V/A) [49]. Modifications induced to the IR spectrum by the application of an oriented external static and homogeneous electric field (Figure 2) take their roots in the interactions established by the external electrostatic potential gradient and the molecular methanol dipoles. As shown in Figure 4, where the molecular dipole moment magnitude is plotted as a function of the field strength, the averaged (over the duration of the AIMD simulations and the number of molecules composing the simulation boxes) methanol dipole moment linearly increases with increasing the strength of the externally applied field. Such a finding is perfectly stackable with that observed in liquid water as well [47]. Figure 1.1 Average molecular dipole moment magnitude as a function of the field strength. Calculated points are plotted as full blue circles whereas the linear fit is presented as dashed blue line. Figure 1.2 IR absorption spectra of bulk liquid methanol determined at 240 K (black line), 270 K (blue line), 300 K (red line), and 350 K (green line) in absence of electric field. Notice that the red-line curve coincides with the red-line curve of Figure 2 and with the black-line curve of Figure 1 (i.e., \(T=300\) K and \(E=0.00\) V/Å). Methanol dipole moments are intimately connected with the behavior of the OH and CO covalent bonds. As for the OH covalent bond, instead of monitoring the distributions of the lengths of such a bond, another very useful quantity has been evaluated. In fact, an indicator capable of monitoring the dissociation degree in H-bonded systems is the proton-sharing coordinate [80; 81].