Miroslav Holík

NMR study of p-disubstituted benzenes with non-coplanar conjugated substituents

Abstract Holik, M., 1993. NMR study of p -disubstituted benzenes with non-coplanar conjugated substituents. Chemometrics and Intelligent Laboratory Systems , 19: 225–231. In the series of 1- X ,4- Y -disubstituted benzenes, where X is a variable substituent (an atom or functional group) and Y is a fixed substituent (a composite group which can conjugate with the benzene ring), the dihedral angle between the benzene ring and the plane of the Y substituent can depend on the variable substituent X . For the series of p -substituted benzylidenediketones ( I ) styrenes ( II ), and N,N -dimethylbenzamides ( III ) the dihedral angle decreases with increasing donor character of the X substituent (type DID). If the part of the Y substituent with a large diamagnetic anisotropy effect (e.g., a CO group) moves in the vicinity of the C3 carbon atom, as in series I , then the C3 chemical shift can be described by a two-variable equation of 13 C NMR substituent chemical shifts with z m , and Hammett σ para constants of the X substituent as explanatory variables. For the series II and III the correlation of C3 chemical shifts with only z m constants is adequate.

NMR chemical shifts in correlation analysis

Abstract Substituent 13 C NMR chemical shifts of monosubstituted benzenes, i.e., substituent shift increments (SSI), z j , can be used as explanatory variables in multivariable regression of aromatic 13 C NMR chemical shifts in the series of para-disubstituted benzenes possessing one fixed ( Y ) and one variable ( X ) substituents. For the substituents Y acting as electron donors or acceptors or affecting the magnetic field around near nuclei by their diamagnetic anisotropy some regression parameters b j in equations for δ C1 and δ C3 account for these phenomena. The effect of the number of points (substances) and accuracy of SSI was investigated with regard to the statistical significance of the regression parameters.

Calculation of the tilts of curved lines

Abstract From time to time, we encounter some dependence in a form of a curve. For the sake of comparison, the tilt of the curve could be taken as its characteristics. We present two methods for calculating a ratio of tilts of the curved lines. The first one consists in singular value decomposition of the data matrix. If the variability in the data is caused by single effect, i.e., the first singular value is about 99 times larger than the second one, then the first loadings vector reflects the ratio of tilts. As the useful variants of this method, the evolving and gliding calculation on the reduced data matrices were investigated; if they give the same (overlapping) results the method is considered as adequate. The other method comprises calculating suitable cosine and sine functions of amplitudes, respectively. In this cos–sine method each vector, and consequently each curve, is represented by two coordinates (xf, yf) of a point and the whole data matrix can be visualised as a cluster of points which can be circumscribed by confidence ellipses. If the shorter half-axis in such an ellipse is negligible, then the norms of the point vectors are in ratio of curve tilts.

Spectroscopic investigations of β-dicarbonyl compounds: II. Structure of anomalous product from the formylation of 2-methylbenzoxazole ethoiodide

Abstract On the basis of NMR, UV, IR and mass spectra the structure of 4,7-dihydro-2,3-benzo-4-ethyl-6-formyl-7-oxo-1, 4-oxazepine was established for the product of the formylation of 2-methylbenzoxazole ethoiodide.

Determination of the Accurate Values of the Rate Constant and Thermodynamic Parameters for the Rotation About the C(sp2)-C(aryl) Bond; Adamantan-1-yl 3-Bromo-2,4,6-trimethylphenyl Ketone

Four programs for the 1 H NMR line shape analysis: two commercial - Winkubo (Bruker) and DNMR5 (QCPE 165) and two written in our laboratory - Newton (in Microsoft Excel) and Simtex (in Matlab) have been tested in order to get highly accurate rate constants of the hindered rotation about a single bond. For this purpose four testing criteria were used, two of them were also developed by us. As supplementary determinations the rate constants obtained for the coalescence temperature and for the thermal racemization of chromatographically separated enantiomers were used which fitted well the temperature dependence of the rate constants determined by the line shape analysis. As a test compound adamantan-1-yl 3-bromo-2,4,6-trimethylphenyl ketone was prepared and studied. It was shown that supermodified simplex method used in our algorithm (Simtex), though time consuming, gives the most accurate values of the rate constants and consequently the calculated thermodynamic parameters E a , ∆ H ≠ , and ∆ S ≠ lay in relatively narrow confidence intervals.