Acetonitrile molecules

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

Polar + Nonpolar Mixtures Figure 7.3 summarizes Hi G= and for mixtures of acetonitrile with benzene at 318.15 K.2 The large positive H results principally from the energy that must be added to separate the highly polar acetonitrile molecules, (which are held together by strong (dipole + dipole)... 

On the other hand, however, Sajeevkumar and Singh found the average coordination number to be 4.7 (141), whereas Megyes et al. found up to six acetonitrile molecules bound to the Li+ ion in the gas phase by means of mass spectrometry (142). Therefore, we reinvestigated the maximum coordination number of [Li(CH3CN)n]+ and also of [Li(NCH),J 1 purely by quantum chemical calculations, which resulted in [Li(NCH)4]+ and [Li(CH3CN)4]+ (93). 

On application of the NMR method mentioned above for the solvent mixture consisting of acetonitrile (DAT —14.1) and nitromethane (DN = 2.7), it was found that also in this case the Li+ ion is coordinated bv four acetonitrile molecules (Fig. 3). 

Surprisingly, on reacting [Fe(NCMe)e]2+ with unsaturated tetraaza-macrocycles similar to those described above, but containing 15 or 16 atoms in the ring, the two axially positioned acetonitrile molecules from the reagent become electrophilically bound to the apical carbon atom of the macrocycle giving rise to new, low spin, bis(macrocycle)hexadentate Fe(II) complexes, Scheme 11. 

The solid state structure determination of 12d undoubtedly proved the iiii configuration of the molecule, which presents a pseudo-C4 symmetry with the P=0 bonds directed inward (Fig. 1) [66]. The crystal contains six solvent molecules per host. One acetonitrile molecule was found as a guest in the aromatic cavity. At the lower rim, a second molecule of acetonitrile occupied the position between the four long chain substituents. 

While anion-2 is clearly the dimer radical anion of acetonitrile, identification of anion-1 as a cavity electron requires caution. First, we stress that anion-1 cannot be the monomer anion of acetonitrile. The monomer anion does not absorb in the NIR [30,46,52]. For the monomer anion to occur at all, the neighboring acetonitrile molecules should all be oriented in the same direction, as in p-acetonitrile otherwise, coupling to a neighboring (antiparallel) molecule reduces the overall energy and causes instant dimer formation. It is difficult to see how such a fortuitous orientation could persist for 0.3-3 nsec in a room temperature liquid. Also, it is not clear why a monomer anion would... 

For linear and quasi-linear, such as a 3-site model for acetonitrile, molecules, only two angles instead of three hsted above are needed to specify the molecular orientation. Eq. (21) gives... 

The acetonitrile molecules are labile and can be exchanged for other mono- or bidentate ligands such as pyridine or tmeda. With N-(2-dimethylamino)ethylmor-pholine two diastereomers are obtained in excellent yield in a ratio of 3 2 (de 20%), one of which could be characterized by X-ray diffraction analysis. 

The adsorption isotherm of N, on FSM-16 at 77 K had an explicit hysteresis. As to the adsorption hysteresis of N-, on regular mesoporous silica, the dependencies of adsorption hysteresis on the pore width and adsorbate were observed the adsorption hysteresis can be observed for pores of w 4.0nm. The reason has been studied by several approaches [5-8]. The adsorption isotherm of acetonitrile on FSM-16 at 303K is shown in Fig. 1. The adsorption isotherm has a clear hysteresis the adsorption and desorption branches close at PIP, = 0.38. The presence of the adsorption hysteresis coincides with the anticipation of the classical capillary condensation theory for the cylindrical pores whose both ends are open. The value of the BET monolayer capacity, nm, for acetonitrile was 3.9 mmol g. By assuming the surface area from the nitrogen isotherm to be available for the adsorption of acetonitrile, the apparent molecular area, am, of adsorbed acetonitrile can be obtained from nm. The value of am for adsorbed acetonitrile (0.35 nnr) was quite different from the value (0.22 nm2) from the liquid density under the assumption of the close packing. Acetonitrile molecules on the mesopore surface are packed more loosely than the close packing. The later IR data will show that acetonitrile molecules are adsorbed on the surface hydroxyls in... 

It is well known that the v, band of liquid acetonitrile is significantly asymmetric due to an overlap of hot band transitions in the low frequency side. A study of gas phase rotation-vibration spectrum [19] showed that the hot band transition from the first exited state of the degenerated C-C = N bending v8 mode, v hl = v + v8 - vs, has its center at 4.944 cm 1 lower than that of the fundamental transition, v,. Also the presence of v,h2 = v, + 2v8 - 2v8 transition is expected. The careful study on the v band of liquid acetonitrile by Hashimoto et al [20] provided the reorientational and vibrational relaxation times of liquid acetonitrile molecule. They corrected the contribution by the hot band transition using the Boltzmann population law and approximated the v , v,hl, 2h2, and v, + v4 bands by Lorentzian curves. 

Therefore, data of Fig. 6 show the change of the reorientational-vibrational relaxation time of acetonitrile molecules confined in mesopores upon adsorption and desorption. Before the capillary condensation, the relaxation time is smaller than that of bulk liquid, whereas it is greater than that of the bulk liquid after condensation. The difference of molecular motion between precondensation and postcondensation states is not significant, but this work can show clearly the presence of such a difference. If vibrational and reorientational relaxation processes are dominated by molecular collisions, the molecular reorientation is more rapid before condensation and it becomes slower than that of the bulk liquid with the progress of the capillary condensation, which indicates the formation of a weakly organized molecular assembly structure in mesopores. Even the mesopore can affect the state of the condensates through a weak molecular potential. The organized state should be stable in mesopores, because the relaxation time is almost constant above the condensation PIP,. 

Another example of a system with (polar + nonpolar) interactions is shown in Figure 17.7, where curve 1 shows for mixing non-polar tetrachloro-methane molecules with polar acetonitrile molecules.10 The breaking of (dipole + dipole) interactions in the acetonitrile when mixed with tetrachloro-methane results in a large positive H... 

---