Formamide resonance

Two Lewis structures (resonance forms) of formamide the atoms are connected in the same order but the arrangment of the electrons is different... 

The reaction of eq. 16.9 will regenerate the antioxidant Arj-OH at the expense of the antioxidant At2-OH. Despite the fact that such regeneration reactions are not simple electron transfer reactions, the rate of reactions like that of eq. 16.9 has been correlated with the E values for the respective Ar-0. Thermodynamic and kinetic effects have not been clearly separated for such hierarchies, but for a number of flavonoids the following pecking order was established in dimethyl formamid (DMF) by a combination of electrolysis for generating the a-tocopherol and the flavonoid phenoxyl radicals and electron spin resonance (ESR) spectroscopy for detection of these radicals (Jorgensen et al, 1999) ... 

Protons on carbon geminal to -NCS or -NHCHO functions do not experience the weak coupling with nitrogen due to the normal hybridization of the nitrogen atom. In the case of formamides, however, both E- and Z-rotamers are possible as revealed by resonances in the H NMR spectrum. 

A minor isonitrile named acanthellin-2 with a specific rotation of — 24.1° was reported with no further details except some IR and MS data [3]. Isothiocyanate 15 and formamide 16 were also isolated from Axinella cannabina in a later study, in which isonitrile 14 and isomers 17-19 were also secured [35]. That 17-19 are C4(14)-ene isomers of 14 was concluded after extensive double resonance H NMR experiments. Coupling constants and nOe effects between the affected protons and various substituents of 17 established the relative stereochemistry of this series. 

Such one-center enhancement effects can be illustrated by formamide 5 for nb—>7Ta (3.109c) interactions. As shown in Table 3.19, the n — -7tco interaction of 5 leads to strong conjugative stabilization (59.8 kcal mol-1) and reduced C—O bond order (1.732), the famous amide resonance of peptide chemistry ... 

The RAHB effect may be illustrated by the ubiquitous C=0- -H—N hydrogen bond of protein chemistry. As shown in Section 5.2.2, the simplest non-RAHB prototype for such bonding, the formaldehyde-ammonia complex (5.31c), has only a feeble H-bond (1.41 kcalmol-1). However, when the carbonyl and amine moieties are combined in the resonating amide group of, e.g., formamide, with strong contributions of covalent (I) and ionic (II) resonance structures,... 

Further synergistic enhancement of amide resonance and H-bonding occurs when both monomers can participate in two complementary H-bonds, once as a Lewis base and once as a Lewis acid. Such concerted (cooperative) pairs of H-bonds occur in the cyclic formamide dimer, as illustrated in Fig. 5.20. In this case the strength of each H-bond is further enhanced (to 6.61 kcalmol-1, about 4.7 times that of the prototype (5.31c)), the bond orders ben and bco are further shifted (to 1.384 and 1.655, respectively), and the bond lengths undergo further shifts in the... 

Hindered rotation can separate the NMR signals of apparently equivalent protons. This causes changes in the geometry of the molecule. This fact is illustrated by C - N bond if N, N dimethyl - formamide which has partial double bond character due to resonance stabilization of the molecule ... 

PMR, proton magnetic resonance IR, infrared DMSO, dimethyl sulfoxide DMF, dimethyl-formamide. 

Formyl complexes are characterized by distinct spectroscopic features. The most frequently used diagnostic probe has been H NMR. Chemical shifts in the 8 12-17 range (several ppm downfield from normal aldehyde or formamide H-NMR resonances) (69) are observed. However, since chemical shifts measured in the presence of trialkylborane by-products can show marked temperature dependence (32, 38, 47, 66), absolute values must be treated with caution. 

Several structural features of 39 suggest that resonance structure 51b significantly contributes to its ground state. First, the formyl C=0 distance, 1.220(12) A, is as long or longer than the corresponding C=0 bond distance in formamide (1.193 A) (74). Second, the plane of the formyl... 

The proton-decoupled 15N spectrum of formamide is shown at the top of Figure 6.2. This spectrum looks remarkably like a 13C spectrum with a single resonance. This 13C-like appearance as opposed to an H-like... 

The same methodology was applied to the study of the role of conjugation in the stability and rotational barriers of formamide and thioformamide [100]. Here it was found that resonance accounts for roughly one-half of the rotational barrier of formamide and for two-thirds in the case of thioformamide. 

An understanding of the internal rotation about the amide bond is important because of its relevance to protein structure. Formamide is the simplest amide. The coplanarity and the remarkable rotational barrier about the C-N bond in formamide can be rationalized by resonance between the n electrons of the carbonyl group and the lone pair of the nitrogen atom [1, 50]. According to VB theory, the Jt electronic structure of formamide may be described by six resonance structures. 

Here we revisit the VB resonance model in formamide by taking all six resonance structures into account. Such a study allows us to compare the individual contributions from resonance structures 1-6 to the resonance effect in formamide. For comparison, the isoelectronic systems vinylamine and formamidine are also investigated to gain insights into the trends of resonance stabilization. A 6-31G(d) basis set is employed in the calculations, and the orbitals in the VB functions are self-consistently determined for each resonance structure, but restricted to be atomic orbitals. The structural weights of the six resonance structures are listed in Table 2. 

Energetically, the resonance effect in the three systems decreases in the order of formamide > formamidine > vinylamine, which is consistent with the rule of atomic electronegativity. It is worthwhile to note that Wheland [50] estimated the resonance energy of formamide based on the heats of combustion and gave a... 

Many molecules are represented as a set of resonating structures. For example, the ground state of formamide is the optimized mixture of the VB structures 34 and 35. The resonance energy, which is responsible for the... 

This resonance representation correctly predicts a planar amide nitrogen atom that is sp2 hybridized to allow pi bonding with the carbonyl carbon atom. For example, formamide has a planar structure like an alkene. The C—N bond has partial double-bond character, with a rotational barrier of 75 kJ/mol (18 kcal/mol). 

Template reactions have been monitored by NMR spectroscopy (126) for the reaction of Na + ions with dibenzo-18-crown-6 in TV,N-dimethyl-formamide. One drawback of this technique is that the presence of paramagnetic ions, i.e., many transition metal ions, would cause appreciable broadening of the resonances. 

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