The Raman spectra of the phenyl esters of cinnamic and benozic acids have been studied and those of the methyl and ethyl esters of the above acids along with that of phenyl acetate have been reinvestigated. From the above data and those obtained by other workers relating to alkyl and phenyl esters and some other carbonyl compounds the C=O frequencies have been tabulated. It is noted that the phenyl group attached directly to the carbon of the C=O group as in esters of aromatic acids and in aromatic ketones, etc., markedly lowers the carbonyl frequency whereas the same group attached to the oxygen atom of the COO group definitely enhances the frequency. These facts are explained from considerations of electromeric polarisation and resonance.

Raman spectra of esters containing the benzyl group, (i) benzyl esters of various acids and (ii) esters of phenyl acetic acid derived from different alcohols, have been investigated. Data relating to other carbonyl compounds such as ketones, aldehydes and acid chlorides have been collected. In regard to its influence on the C=O frequency the benzyl group is found to resemble closely alkyl groups and to differ markedly from the phenyl. This confirms the views already expressed in Part II and establishes that the C=O group is influenced mainly by the electromeric effect. The introduction of an additional CH₂ prevents the transmission of this effect.

The Raman spectra of cinnamyl alcohol and cinnamyl acetate are re-investigated and their frequencies compared with those of ethyl cinnamate. A great similarity between the spectra of all these substances is observed. The higher C=C and C=O frequencies in cinnamyl acetate and allyl acetate as compared with ethyl cinnamate and ethyl acrylate is explained as due to the existence of resonance in the latter. It is pointed out that when comparisons are effected carefully the C=C and C=O bonds mutually weaken each other when they exist in conjugation.

It was shown that a solution of ferric chloride (0·08 N) could be satisfactorily employed as a permanent standard in the colourimetric estimation of nitrate by the α-naphthol sulphonic acid method. The close resemblance of the colours of the ferric chloride and the corresponding standard nitrate solution was established by analysing them making use of the Keuffel and Esser colour analyser.

The analysis of colours that are met with in the colorimetric estimation of nitrate by the phenol sulphonic acid and the α-naphthol sulphonic acid reagents has been effected by means of the Keuffel and Esser colour analyser. From the results of colour analysis as well as those of colorimetric estimations it has been shown that the α-naphthol sulphonic acid method and the phenol sulphonic acid method are both complementary, each adding to the utility of the other by being efficient in its own range of concentration.

A brief survey of the existing literature on hydrogen bonds is made. The advantages of employing the Raman effect for investigations in this field are discussed. The results not only supplement those obtained from infra-red studies but they become more definite, particularly when such groups as the C=O, which yield sharp Raman lines, are employed.

Hydrogen bond formation between various esters as donor molecules and phenol, alcohols and chloroform as acceptor molecules has been studied and the results are presented in this paper. The changes in the C=O frequencies are classified. It is shown that there are large variations in the strengths of hydrogen bonds. The generalization that the strengths of hydrogen bonds depend on the anionoid power of the donor and the cationoid power of the acceptor atoms is made as a logical consequence of the mechanism of hydrogen bonds and is supported by the experimental results that are recorded.

The Raman spectra of a few typical aldehydes and ketones and their mixtures with acceptor solvents such as phenol, methyl and ethyl alcohols and chloroform are recorded and the results are discussed. They support the general conclusion arrived at in Part I that large variations in hydrogen bond strengths exist and that they depend upon two factors (1) the anionoid power of the donor atom and (2) the cationoid power of the acceptor atom. The question of distinguishing between two types of hydrogen bonds, the hydrogen and hydroxyl bonds, as suggested by Bernal is discussed in the light of our results. It is shown that there is justification for division into two categories and a proposal is made to effect the classification as below: (1) True Hydrogen Bonds. (2) Hydrogen Bonds. In Class (1) both the cationoid (C=O) and anionoid (OH) centres are affected whereas in class (2) only the cationoid group indicates change.

The influences of constitutive factors on the capacity of the carbonyl compounds to form hydrogen bonds are explained as due to their effect on the strength of the C=O group. Similar explanation has already been given in Part I regarding the behaviour of the solvents containing cationoid atom (H atom) in hydrogen bond formation.

Previous work on the Raman spectrum of acetic acid and of its solutions is critically reviewed and certain conclusions criticised. The Raman spectra of the acid in the pure state as well as in aqueous solutions at different dilutions have been reinvestigated and the results described. A comprehensive interpretation of the observations is offered. It is shown that the breaking up of the ring dimers of the acid is brought about by the formation of hydrated molecules, and that the formation of fresh hydrogen bonds between the acid and water molecules is the cause of this breakdown of the ring type into the open dimers and finally into hydrates of the monomeric form of the acid molecules. These ideas are supported by a study of the Raman spectrum of a mixture of acetic acid with a good acceptor solvent, phenol. Changes similar to those obtained in aqueous solutions are produced in the C=O frequencies of the phenol mixture also.

Raman spectra of mixtures of acetic acid with a number of carbonyl compounds have been studied. It is shown that anionoid solvents affect the molecular aggregation of acetic acid, the effect being prominent with those having strong anionoid C =O groups such as coumarin and phenyl acetate. The breakdown of the ring dimers is not, however, complete in these cases. Definite evidence has been given for the presence of open dimers. There is justification for the view that water molecules serve also as anionoid molecules in effecting the breakdown of the ring dimers of acetic acid.

Raman spectra of mixtures of acetic acid with donor solvents not having C=O groups have been studied. The solvents chosen are dioxan and acetal. The results lend definite support to the mechanism described in previous papers (Parts III² and IV¹) of the progressive breakdown of the ring dimers of the acid into associates of the open dimers and eventually of the monomers with the solvent molecules. The extraordinary efficiency of water in bringing about the breakdown, is attributed to its dual capacity to act as both donor and acceptor.

The Raman spectra of methyl, ethyl,n-butyl, iso-amyl and phenyl esters of salicylic acid have been studied. Besides the bright frequency attributable to the C=O group of the chelate structure, a weaker line corresponding to the unchelate molecule has been observed in each case. Thus the liquids contain the two types existing in equilibrium, the chelate molecules being predominant. The extent of the lowering of the C=O frequency brought about by chelation indicates that this type of intamolecular hydrogen bond is strong. This is also supported by the study of the mixtures of methyl and ethyl salicylates, with chloroform, phenol, dioxan and acetic acid. The spectra are unaffected showing that the chelate bond cannot be broken by anionoid and cationoid solvents and that salicylates cannot function as other esters and phenols do in breaking down hydrogen bonds.

Raman spectra of (1) propionic acid, (2) formic acid, (3) benzoic acid and (4) cinnamic acid have been studied using the pure acids in the liquid or molten condition and solutions in various solvents, benzene (non-associating), water, ether dioxan, acetal, etc. (associating). The first closely resembles acetic acid both of them indicating the existence of ring dimers, open dimers and monomers, the last two forms being present to a very small extent in the free state and in increasing quantities (in association with solvents) in solution in associating liquids. The state of the molecules in formic acid seems to be more complex due probably to the capacity of H of the C−H group to form hydrogen bonds. There are 2 broad C=O lines equally strong in the pure liquid at 1670 and 1715 cm.⁻¹; the former diminishes and the latter increases in solutions with associating solvents. The narrowing of the band at 190 cm.⁻¹ is another characteristic. It seems to be possible that in this acid no molecular type having unco-ordinated C=O exists. Benzoic and cinnamic acids form a different type. The molten acids and benzene solutions give only one C=O line corresponding to ring dimers and in fairly dilute solutions in ethers monomer-associates alone containing unco-ordinated C=O groups exist. Thus they do not give indications of the intermediate open type of dimers.

The marked solubility of coumarin in aqueous mineral acids is attributed to the formation of complexes. That this takes place through hydrogen bonds is shown by a study of the Raman spectrum of these solutions wherein the carbonyl frequency is found to be considerably lowered, but still persists.

The Raman spectrum of salicylic acid has been studied for the first time in dioxan and benzene solutions. In the former monomolecular chelate structures associated with the solvent predominate; in the larter some chelate monomers exist, but the major portion consists of dimers whose structure is not quite clear. The spectrum of acetyl salicylic acid (aspirin) in dioxan seems to indicate that it has a chelate structure.

Some systematic studies have been carried out on the influence of temperature on the photometric determination of silicon by the molybdenum blue procedure. It has been shown that temperature exerts a distinct influence on the colour intensities involved in the different procedures and the extent of this influence depends largely on the nature of the acid medium and the reducing agent employed. Ascorbic acid has been successfully employed to overcome the influence of temperature.

Our thanks are due to the management of the Tata Iron and Steel Company for permitting the publication of this paper.