Nature of substituent effects in nuclear magnetic resonance spectroscopy. 1. Factor analysis of carbon-13 chemical shifts in aliphatic halides

Abstract

The 13C spectra of 18 sets of aliphatic and alicyclic halides (F, C1, Br, I) have been obtained as 2 M solutions in C'DC13 and have been assigned. The differences in chemical shift with respect to the parent hydrocarbons (Ah) have been analyzed by the use of factor analysis. This technique leads to two sets of parameters: "intrinsic" substituent effects (a values) characteristic of the halogens and attenuation parameters (b values) which reflect the sensitivity of each molecular position to the substituent effects. In the present case it was found that only three-substituent-effect terms for each halogen are needed to correlate all of the data. The chemical shifts are related to the parameters by A6, = axlb, + a&, + ax3bs where x refers to the halogens and i refers to the molecular positions. The first substituent parameter (al) is essentially a constant for all of the halogens and appears to be related to the polar effect of halogen v i s -h i s hydrogens. The corresponding attenuation parameters (b,) represent the sensitivity of each molecular position to polar effects of the halogens. The second substituent parameter (az) increases approximately linearly on going from fluorine to iodine, controls the differences in A6for the series of halides, and appears to be related to the "freeness" of the valence electrons about the halogens. The third parameter (a3) is considerably less significant than the others. An analysis of the effect of the structural changes on the molecular attenuation parameters (bl-b3) reveals that the two most important substituent parameters are each transmitted to the a, 0, and y positions via the same or linearly related mechanisms. The a2 halogen factors are linearly related to a variety of non-NMR parameters characteristic of the halogens (reciprocals of the ionization potentials of CH&, wavelength of the first electronic transition of CH,X, C-X bond polarizabilities, and atomic contributions to molar diamagnetic susceptibilities) as well as the diamagnetic and paramagnetic susceptibiliities derived from NMR measurements.

Poly(vinyl chloride) and wood flour press mould composites: New bonding strategies

“…35,36 The active chain ends can be identified by the peak at around 17 ppm. 37 The reduction of the peak from PHPA to PVC-PHPA can be explained by the growth of the copolymer molecular weight through reaction of the former active chain ends with VCM. Chlorine microanalysis indicates a total PVC incorporation of 10.2 % (wt.)…”

“…Molecular relaxations, such as a relaxations (or glass transitions), b relaxations, and the peak of tan d, are always frequency dependent. 37 Examples of processes that are not frequency dependent are melting, thermal degradation, curing and crystallization. Figure 12 shows the DMA thermograms of the PVC that was used in this work.…”

13C NMR substituent effects in aliphatic systems: Principal component analysis and correlations with electronegativities

“…An intriguing alternative can be found in the use of factor or principal component analysis (PCA),24,25 which could 'allow the data to speak for themselves'. 26 Multivariate analytical techniques can, in principle, extract the minimum set of independent descriptors for a large set of data, and are therefore increasingly independent descriptors for a large set of data, and are therefore increasingly applied in many branches of ~cience. '~ The automatic analysis of NMR shielding data, which are readily obtained in large numbers, should further represent a significant step towards the introduction of artificial intelligence in chemistry.…”

“…26 concluded from an investigation with 18 sets of aliphatic halides that the inductive influence remains constant throughout the series of halogens, and that another factor, connected to the 'freeness' of halogen valence electrons or, for example, to their ionization potentials, polarizibilities, etc., determines the carbon shift differences between the halides. This result seems to be at variance with the many known correlations of substituent electronegativity, particularly with C-a shifts (see also Fig.…”

Determination of substituent effects by factor analysis and multiple linear regression for the carbon-13 nuclear magnetic resonance chemical shifts in 4-substituted phenols and 2-nitrophenols

“…The use of factor analysis and principal component analysis has only recently been used to explore the effect of substituents on I3C chemical shifts (17)(18)(19)(20)(21). Reynolds et al (22) have used factor analysis on their extensive 3-and 4-substituted styrene data to show that the DSP procedure is necessary to fit the para chemical shifts, and that a third parameter is required for each of the ortho, meta, and ipso carbons in the styrene derivatives.…”