Nitromethane, dipole moment

The rate enhancement of the Diels-Alder reaction in LP-NM has been attributed to the high dipole moment of nitromethane (3.40 D) in comparison with diethyl ether (1.33 D). 

Parker37 defined class 4 as solvents "which cannot donate suitable labile hydrogen atoms to form strong hydrogen bonds with an appropriate species and proposed the designation dipolar aprotic solvents he extended their range down to s > 15 and quoted as examples acetone, acetonitrile, benzonitrile, dimethylformamide, dimethyl sulphoxide, nitrobenzene, nitromethane (41.8) and sulfolane (tetramethylene sulphone) (44), where e varies from 21 to 46.5, and the dipole moment p from 2.7 to 4.7 debye. 

The dipole moment is a fundamental property of a molecule (or any dipole unit) in which two opposite charges are separated by a distance . This entity is commonly measured in debye units (symbolized by D), equal to 3.33564 X 10 coulomb-meters, in SI units). Since the net dipole moment of a molecule is equal to the vectorial sum of the individual bond moments, the dipole moment provides valuable information on the structure and electrical properties of that molecule. The dipole moment can be determined by use of the Debye equation for total polarization. Examples of dipole moments (in the gas phase) are water (1.854 D), ammonia (1.471 D), nitromethane (3.46 D), imidazole (3.8 D), toluene (0.375 D), and pyrimidine (2.334 D). Even symmetrical molecules will have a small, but measurable dipole moment, due to centrifugal distortion effects. Methane " for example, has a value of about 5.4 X 10 D. 

An example of reaction type (c) in Table 5-4 is the well-known Menschutkin reaction [30] between tertiary amines and primary haloalkanes yielding quaternary ammonium salts. Its solvent dependence was studied very thoroughly by a number of investigators [51-65, 491-496, 786-789]. For instance, the reaction of tri-n-propylamine with iodomethane at 20 °C is 120 times faster in diethyl ether, 13000 times faster in chloroform, and 110000 times faster in nitromethane than in -hexane [60]. It has been estimated that the activated complex of this Menschutkin reaction should have a dipole moment of ca. 29 10 Cm (8.7 D) [23, 64], which is much larger than the dipole moments of the reactant molecules (tris- -propylamine 2.3 10 Cm = 0.70 D iodomethane 5.5 10-3 1 64 D) [64]. 

Sn(CH3)3l dissolved in nitrobenzene as a function of concentration of various EPD solvents added (35). In noncoordinating or weakly coordinating solvents, such as hexane, earbon tetrachloride, 1,2-dichloroethane, nitrobenzene, or nitromethane, Sn(CH3)3l is present in an unionized state (tetrahedral molecules). Addition of a stronger EPD solvent to this solution provokes ionization, presumably with formation of trigonal bipju amidal cations [Sn(CH3)3 (EPD)2J. Table II reveals that the molar conductivities at a given mole ratio EPD Sn(CH3)3l are (with the exception of pyridine) in accordance with the relative solvent donicities. No relationship appears to exist between conductivities and the dipole moments or the dielectric constants of the solvents. 

Closely related to the ideas of bond polarity and dipole moment is the occasional need to assign formal charges to specific atoms within a molecule. This is particularly common for atoms that have an apparently "afanormal number of bonds. In nitromethane (CH3NO2), for example, the nitrogen atom has four bonds rather than the usual three and has a formal positive charge. 

Using the data given below for dilute solutions of nitromethane in benzene, calculate the molar polarization and refraction of the solute in the limit of infinite dilution. Then, estimate the dipole moment of nitromethane on the basis of the Debye model. 

The complications due to ion pairing in non-aqueous solvents can be avoided to some extent by the use of dipolar aprotic solvents which have been much investigated recently. These solvents are unable to act as hydrogen bond donors (i.e., they have at most very weak acid properties), but they have large dipole moments and polarizabilities, and hence moderately high dielectric constants. Examples are acetonitrile (e = 38), dimethyl sulphoxide (s = 49), sulpholane s = 38), nitrobenzene (e = 35), nitromethane (6 = 36), dimethylformamide (e = 38), and propylene carbonate (6 = 65). Some of them can act as bases and hydrogen bond acceptors, but this is not true of nitrobenzene and nitromethane. Unlike the solvents which we have considered so far, they show little specific tendency to solvate anions, which has important consequences for the reactivity of many anions towards organic compounds. ... 

AG° values are generally quite small(34-36). In Table II are given Ago values for a number of nonpolar and other solutes to illustrate the trends the Ag values for t-butyl chloride are quite close to those for n-pentane, a nonpolar solute of almost the same size as t-butyl chloride. However, the values of Ag for the transition state. Table I, do not resemble those for nonpolar or slightly polar solutes (nitromethane has a dipole moment of 3.5 D) clearly more polar species need to be examined as models for the transition state. Some years ago(24,29), the ion-pair Me N Cl was suggested as a suitable model, and more recently, AG values for the transition state have been compared to those for zwitterionic a-aminoacids(35). The relevant AG° values are collected in Table III. 

Closely controlling the quality of our basis set and its effects on our states will confirm the validity of our results. Let us illustrate this aspect with another example. Neutral nitromethane (CH3NO2) has a dipole moment of 3.46 D. A standard ANO-L C,N [4s3p2d]/H[3slpld] basis set is used supplemented with a set of Is diffuse functions (eight primitives) with exponents for the diffuse functions explicitly optimized to deal with Rydberg states. At the CASSCF optimized... 

Figure I-B-11 shows plots of the vapor pressures of methyl nitrite, nitromethane, and methyl nitrate as a function of reciprocal temperature. It is seen that at a given temperature the vapor pressures decrease in the order CH3ONO > CH3ONO2 > CH3NO2. This is the order of decreasing polarity of the molecules as measured by the calculated dipole moments (/r) /U.CH3NO2 (3.4 D) /U.CH3ONO2 (2.9 D) > /tchsONO (2.2 D), and it seems likely that dipole-dipole interactions are important in determining the volatility of these compounds.

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