Frequency: stretching

Hammen equation A correlation between the structure and reactivity in the side chain derivatives of aromatic compounds. Its derivation follows from many comparisons between rate constants for various reactions and the equilibrium constants for other reactions, or other functions of molecules which can be measured (e g. the i.r. carbonyl group stretching frequency). For example the dissociation constants of a series of para substituted (O2N —, MeO —, Cl —, etc.) benzoic acids correlate with the rate constant k for the alkaline hydrolysis of para substituted benzyl chlorides. If log Kq is plotted against log k, the data fall on a straight line. Similar results are obtained for meta substituted derivatives but not for orthosubstituted derivatives. 

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

Of course, the guesses above aren t really guesses. They are predicated on many years of Raman and other spectroscopic experience and calculations that are the reverse of the calculation we descr ibed. In spectroscopic studies, one normally calculates the force constants from the stretching frequencies in modeling, one... 

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

All of the calculated vibrational frequen cies given on Learn mg By Modeling are too high For example the C=C stretching frequency of 1 hexene observed at 1640 cm IS calculated to be at 1857 cm" ... 

The C=0 stretching frequency in 2 hexa none appears at 1720 cm To view this VI bration on Learning By Mod elmg select the calculated value of 1940 cm ... 

The S—H stretching frequency of thiols gives rise to a weak band m the range 2550-2700 cm ... 

Section 16 18 An H—C—O—C structural unit m an ether resembles an H—C—O—H unit of an alcohol with respect to the C—O stretching frequency m its infrared spectrum and the H—C chemical shift m its H NMR spectrum Because sulfur is less electronegative than oxygen the H and chemical shifts of H—C—S—C units appear at higher field than those of H—C—O—C... 

Increased single bond character in a carbonyl group is associated with a decreased carbon—oxygen stretching frequency Among the three compounds benzaldehyde 2 4 6 trimethoxybenzaldehyde and 2 4 6 trinitrobenzaldehyde which one will have the lowest frequency carbonyl absorption" Which one will have the highest" ... 

Section 22 19 The N—H stretching frequency of primary and secondary amines appears m the infrared m the 3000-3500 cm region In the NMR spectra of amines protons and carbons of the type H—C—N are more shielded than H—C—O... 

Nonterminal 2260-2150 (var) Symmetrical or nearly symmetrical substitution makes the C=C stretching frequency inactive. When more than one C=C linkage is present, and sometimes when there is only one, there are frequently more absorption bands in this region than there are triple bonds to account for them. 

Diketones, syn-trans-open chains 1730-1710 Antisymmetrical stretching frequency of both... 

The general trends of structural variation on the position of C=0 stretching frequencies may be... 

Hydrogen bonding to a carbonyl group causes a shift to lower frequency of 40 to 60 cm k Acids, amides, enolized /3-keto carbonyl systems, and o-hydroxyphenol and o-aminophenyl carbonyl compounds show this effect. All carbonyl compounds tend to give slightly lower values for the carbonyl stretching frequency in the solid state compared with the value for dilute solutions. 

Carbonyl stretching frequency. Aldehyde proton, relative to TMS. Carbonyl carbon, relative to TMS. 

CAS name CAS Registry Number Formula X = n Bp, °C Melting point, °C Refractive index, Density at 20°C, g/L stretching frequency, max, cm Enol, %... 

Infrared absorption studies have shown that correlates with an absorption at 3 p.m associated with an OH-stretching frequency (20). Indeed, infrared absorption provides a useful tool for evaluation in rapid production quaUty control. Infrared and other studies show that degradation is caused by proton inclusion in the grown quartz. 

Bridging and terminal nitrogens have been compared ia [Cp2Zr(N2)]2N2. The bridging N2 has a longer N—N distance (0.118 vs 0.1115 nm) and shorter Zr—N distances. In addition, the bridging N2 stretching frequency is very low, 1578 cm (248). 

The iacrease ia reactivity of coordinated N2 has been assumed to be associated with iacreased bond length and decreased stretching frequency. A labeling study has shown that this is an oversimplification. In the protonolysis of [Cp 22 (N2)]2 2 hydraziae produced comes equally from terminal and bridging N2. An iatermediate, such as [86165-22-2] was proposed where the bridging and terminal N2 have become equivalent. 

Spectral Characteristics. The iafrared stretching frequency of the penicillin P-lactam carbonyl group normally occurs at relatively high frequencies (1770 1815 cm ) as compared to the absorptions for the secondary amide (1504-1695 cm ) and ester (1720-1780 cm ) carbonyl groups. 

The infrared carbonyl stretching frequencies of n- and isobutyraldehyde in the condensed phase occur at 1727.6 and 1738.0 cm , respectively (38). The proton nmr spectra of both aldehydes are weU-known (39). 

A variety of instmments are available to analyze carbon monoxide in gas streams from 1 ppm to 90%. One group of analyzers determines the concentration of carbon monoxide by measuring the intensity of its infrared stretching frequency at 2143 cm . Another group measures the oxidation of carbon monoxide to carbon dioxide electrochemically. Such instmments are generally lightweight and weU suited to appHcations requiring portable analyzers. Many analyzers are equipped with alarms and serve as work area monitors. 

Figure 4 IR frequencies (cm ) for keto heterocycles (carbonyl stretching frequencies bracketed frequencies are for C=C stretches)...

IR spectroscopy has been particularly helpful in detecting the presence of keto tautomers of the hydroxy heterocycles discussed in Section 3.01.6. Some typical frequencies for such compounds are indicated in Figure 4. Here again the doublets observed for some of the carbonyl stretching frequencies have been ascribed to Fermi resonance. 

Comparison of UV data for 3-aminoisothiazoles with those of reference compounds confirms that they exist in the 3-amino form. A more recent investigation of 4-aminoisothiazole (76MI41701) using deuterium exchange experiments of the type described in Section 4.01.5.2, and analysis of the symmetric and antisymmetric NH2 stretching frequencies in its IR spectrum, show that this compound also exists in the 4-amino form. 

Table 8 Stretching Frequencies for Exocyclic Double Bonds on Small Rings ...

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