Dipole moment amides

In addition, %-% interactions between aromatic surfaces of base pairs (vertical direction) are believed to account for most of the thermal stability. In contrast, a-helices utilize three networks of hydrogen bonds between amide donors and acceptors (N-H 0=C) as well as the constituent amide dipole moments to guide (vertical direction) and stabilize the single helical configuration. Further stability of this structure in aqueous media is achieved by embedding a-helices inside a hydro-phobic environment, e.g., protein interior or membrane," which leads to stronger interactions when water is excluded. 

Cheam, T. C., and S. Krimm. 1985. Infrared Intensities of Amide Modes in N-methyl-acetamide and Poly(Glycine I) From Ab Initio Calculations of Dipole Moment Derivatives of N-methylacetamide. J. Chem. Phys. 82, 1631-1641. 

Amides possess high dipole moments /a due to the presence of the amide group. Some values are given below. 

One is the steric effect. As in the case of amides, the E conformation is disfavored by the steric effect when R is equal to or larger than a methyl group. The second factor is the dipole moment. The E conformation is much more polar than the Z conformation. Because of these factors, esters usually assume the Z conformation and no E conformation is observed. To observe the latter, a special probe is needed. From the foregoing discussion, it is apparent that if R is hydrogen and R a large group, the E conformation might be present. Thus the E form of tm-butyl formate was detected by H NMR spectroscopy at low temperature (84). The barrier was found to be ca. 11 kcal/mol (85). 

Chromatographic Behaviour of log P = 3 Compounds in Aqueous Dimethylform-amide (DMF). DMF is a solvent with a strong dipole moment (Table 4.2). The same log A values in 50% aqueous DMF can be obtained at a concentration of 26% aqueous acetonitrile for ROH, 44% aqueous acetonitrile for PAH, 37% for RB, and 36% aqueous acetonitrile for ROB. When eluents with equivalent Po, Xe, Xd, or Xn values in 50% DMF were selected, different log A values were predicted in all three cases. 

Carbon dioxide is a symmetrical, linear triatomic molecule (0 = C=0) with a zero dipole moment. The carbon-to-hydrogen bond distances are about 1.16A, which is about 0.06A shorter than typical carbonyl double bonds. This shorter bond length was interpreted by Pauling to indicate that greater resonance stabilization occurs with CO2 than with aldehydes, ketones, or amides. When combined with water, carbonic acid (H2CO3) forms, and depending on the pH of the solution, carbonic acid loses one or two protons to form bicarbonate and carbonate, respectively. The various thermodynamic parameters of these reactions are shown in Table I. 

Theoretical studies [42,43], including estimation of the dipole moment, have strongly supported the original hypothesis behind introduction of the fluoroolefin amide surrogates. 

Coplanarity is required if the dipolar structure 1b is to be significant. An appreciable dipole moment may be expected of amides and, in fact, simple amides have dipole moments in the range 3.7-3.8 debye. (For reference, the carbonyl group has a moment of about 2.7 debye, Section 16-1B.)... 

The example of amides offers us an opportunity to present the problem of mesomeric dipole moments more generally. Let us suppose that the real structure of a molecule is represented as mesomeric between the two limiting structures the dipole moments of these structures can be anticipated as vectors A and B. The effective experimental dipole moment of the compound is then given by the vector equation 5, where p is a measure of relative importance of the two structures. Equation 5 is formally identical with equation 3 and is always valid for mesomeric structures, since these are assumed to equilibrate by... 

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