Tautomerism, hybridization

As a final example of the use of isotopic substitution in the study of metal complexes, we cite the use of as a ligand by Collman, Famham, and Dolcetti ( ), who found what they termed hybridization tautomerism in several cobalt-nitrosyl complexes. From their infrared spectra they inferred a rapid equilibrium between a trigonal-bipyramidal Co(l) species having a linear Co-nitrosyl geometry, and a sqviare-pyramidal Co(lll) species, in which the Co-nitrosyl moiety is bent. 

Mich current interest in the spectroscopy of hydrogen bonded systems attaches to the question of how one might infer the shape of the potential function from the vibrational spectrum of the entity. In this connection Wood and his collaborators have recently made major contributions. They have examined the infrared and Raman spectra of a great number of cations of the form (bPB ), where B and B are nitrogen bases or perdeutero-nitrogen bases, and P is either hydrogen or deuterium. 

When B and B were both trimethylamines ( ) the NH stretching band and the ND stretching band were both singlets. The same behavior obtained also when B and B were trlmethyl-amine and pyridine (55)- When B=B =pyrldine ( ), or substituted pyrldines (5, and E=H, the HHt band was split into a 

ACS Symposium Series American Chemical Society Washington, DC, 1975. 

When B, but both bases were various pyridlnes or quinoline, the NIT band could be characterized as a doublet with an Intensity ratio of the components which varied between 1.2 and zero as the pl difference of the bases varied between zero and eight ( ). The N--N stretching mode in these un-symmetrical ions was not substantially more Intense than that found when B=B. Another remarkable feature of the spectra of this set of unsymmetrical ions was the appearance of a new band in the 500-600 cm region whose frequency Increased when the bridge was deuterium substituted. 

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Stork [1963] completely rejects the Russian accusation that resonance theory as developed by Pauling, Wheland, and Ingold fails both empirically and philosophically. However, it should be noted that these criticisms (in the GDR) of the events in Moscow appeared well after the hype was over in the USSR. Furthermore, as Brush [1999, 268] correctly remarks For chemists in the West, the apparent absurdity of the Marxist ideological critique obscured a significant difference between VB and MO. According to Brush the molecular orbital theory permits a more realistic interpretation than the valence bond (or resonance) theory. However that may be, reflection on resonance hybrids, tautomerism, the riddle of benzene, and such like is still a relatively neglected theme in the philosophy of chemistry. 

Dissolve ca. 0 2 g. of product (I) in cold ethanol, and add with shaking 1-2 drops of dilute sulphuric acid. A deep purple coloration appears at once. This shows that salt formation has occurred on the quinoline nitrogen atom to form the cation (Ha), which will form a resonance hybrid with the quinonoid form tils). [Note that the forms (IIa) and (11b) differ only in electron position, and they are not therefore tautomeric.] If, hoAvever, salt formation had occurred on the dimethylaniino group to give the cation (III), thrs charge separiition could not occur, and the deep colour would be absent. 

In 1955 Boyer d al challenged this formulation, and suggested a static, mesomeric system rather than a dynamic, tautomeric one, with Contributing structures of type 9 and 10 to a symmetrical resonance hybrid, proposing the name -o-dinitrosobenzene for the parent System. This notion, however, raised more problems than it solved,... 

The 1-oxide 3-oxide tautomerism [Eq. (3), p. 4] has been discussed earlier (Sections II and III,C) in connection with the problem of the structure of benzofuroxan. A second type of rearrangement involves the furoxan ring and an adjacent substituent group, and arose out of a suggestion of Bailey and Case that 4-nitro-benzofuroxan might be a resonance hybrid of type (57)-(-> (58), rather than 57. NMR ruled out this possibility the three protons present in... 

There are also some couplings in which hydrazones are formed but for which the azo tautomer is not detectable and probably does not exist. This is the case in some coupling reactions involving methyl groups of aromatic heterocycles (see, for example, 12.48 and 12.49 in Sec. 12.5). Replacement of a methyl proton by an arylazo group (Scheme 12-3) would result in an azo compound containing an sp3-hybridized — CH2 — group (12.1). The latter is less stable than the tautomeric hydrazone (12.2), in which there is a n-n orbital overlap from the heteroaromatic to the aromatic system. 

The construction of novel tetracyclic ring systems 1-142, which can be considered as hybrids of the I e Ira h y dropy r ro I o 12,3 - fo] i ndo le and tetrahydroimidazol[l,2-o]in-dole ring system, has been described by Herranz and coworkers [40]. The exposure of tryptophan-derived a-amino nitrile 1-140 to acidic conditions triggers a stereoselective tautomerization to give 1-142 in quantitative yield (Scheme 1.35). 

The formation of indanthrone and flavanthrone, as well as alizarin, during the alkali fusion of 2-aminoanthraquinone can be explained mechanistically on the basis of the initial loss of a proton. The resulting anionic species can be represented as a resonance hybrid and is also tautomeric (Scheme 6.12). Primary 1-hydroxylation of 2-aminoanthraquinone is probably the first step in the formation of the alizarin by-product (compare Scheme 6.8). Such an attack may initiate the formation of flavanthrone [31 ]. It is also possible to envisage the formation of all three species by a radical mechanism [32]. 

The reaction of tautomeric 3-amino-2//-5,6-dihydrooxazines 111 and 112 with an a-bromoketone was discussed in a previous article <2001CHE1054>. Generally, the sp -hybridized nitrogen of 4/7-5,6-dihydrooxazines is a better nucleophile and all the examples here are of 4/7-5,6-dihydrooxazines or their benzo derivatives. 

It is important to distinguish tautomerism from resonance, a term used to indicate that the properties of a given molecule cannot be represented by a single valence structure but can be represented as a hybrid of two or more structures in which all the nuclei remain in the same places. Only bonding electrons move to convert one resonance form into another. Examples are the enolate anion, which can be thought of as a hybrid of structures A and B, and the amide linkage, which can be represented by a similar pair of resonance forms. 

Within the last decade, ab initio and hybrid quantum-chemical methods were in considerable use in tetrazole chemistry, and the level of calculations significantly improved with extended basis sets used for quite complex polyatomic molecules. During this time, theoretical methods were exploited in the study of several fundamental properties of the terazole ring, such as aromaticity and capability to be involved in various kinds of tautomerism, including the effects of substituents and media on these parameters. It was demonstrated that many physical and physicochemical characteristics of tetrazoles could be successfully estimated by these methods not only for the gas phase but also for the condensed state (solvents, crystals). 

Aldehydes, ketones, carboxylic esters, carboxylic amides, imines and N,A-disubstiluted hydrazones react as electrophiles at their s/ 2-hybridized carbon atoms. These compounds also become nucleophiles, if they contain an H atom in the a-position relative to their C=0 or C=N bonds. This is because they can undergo tautomerization to the corresponding enol as seen in Chapter 12. They are also C,H-acidic at this position, i.e., the H atom in the a-position can be removed with a base (Figure 13.1). The deprotonation forms the conjugate bases of these substrates, which are called enolates. The conjugate bases of imines and hydrazones are called aza enolates. The reactions discussed in this chapter all proceed via enolates. 

Little has been reported on tautomerism of dioxoles and oxathioles. The spectral data for a series of 2-dialkylamino-l,3-oxathiolium salts has been interpreted as favoring the immonium structure (43) (72CPB304). Similarly, the spectra of several oxathiol-2-ylidene derivatives (44) indicate that the dipolar contribution to the resonance hybrid structure is small (73CPB2224). 

Since the suggestion of the sequential QM/MM hybrid method, Canuto, Coutinho and co-authors have applied this method with success in the study of several systems and properties shift of the electronic absorption spectrum of benzene [42], pyrimidine [51] and (3-carotene [47] in several solvents shift of the ortho-betaine in water [52] shift of the electronic absorption and emission spectrum of formaldehyde in water [53] and acetone in water [54] hydrogen interaction energy of pyridine [46] and guanine-cytosine in water [55] differential solvation of phenol and phenoxy radical in different solvents [56,57] hydrated electron [58] dipole polarizability of F in water [59] tautomeric equilibrium of 2-mercaptopyridine in water [60] NMR chemical shifts in liquid water [61] electron affinity and ionization potential of liquid water [62] and liquid ammonia [35] dipole polarizability of atomic liquids [63] etc. 

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