scholarly journals Is (benzene)Cr(CO)3 really more aromatic than benzene?

2017 ◽  
Vol 95 (3) ◽  
pp. 263-270 ◽  
Author(s):  
Amnon Stanger

(benzene)Cr(CO)3 was claimed to be more aromatic than benzene, based on the central 1H Me chemical shifts in dimethyldihydropyrene annulated to it. In this paper, several dihydropyrenes are computationally investigated. NICS-scan methods are used to assess the ring current properties, and NMR calculations are used for obtaining NMR chemical shifts. The parent and four benz-annulated dihydropyrenes show excellent agreement with the experimental results, reported by R. Mitchell in several papers. The effect of annulating the antiaromatic cyclobutadiene was shown to be larger than that of benzene and to change the type of ring currents; while benz-annulated dihydropyrenes maintain local 14-annulene and local benzenic ring currents, the cyclobutadiene-annulated dihydropyrenes show global (and local) ring currents. A NICS-scan study of (benzene)Cr(CO)3 and a NICS-XY-scan study of the (CO)3Cr(C6H4)-annulated to dihydropyrene show that the benzene in the Cr complex is at most aromatic as benzene, probably somewhat less. A detailed study of the 1H chemical shifts suggests that the experimental results were somewhat misinterpreted, and that (benzene)Cr(CO)3 is at most as aromatic as benzene.

1984 ◽  
Vol 106 (20) ◽  
pp. 5974-5978 ◽  
Author(s):  
Ronald F. Childs ◽  
Michael J. McGlinchey ◽  
Aravamuthan Varadarajan

2011 ◽  
Vol 115 (13) ◽  
pp. 2830-2836 ◽  
Author(s):  
Sung Soo Park ◽  
Yong Sun Won ◽  
Woojin Lee ◽  
Jae Hong Kim

2013 ◽  
Vol 12 (03) ◽  
pp. 1350009 ◽  
Author(s):  
MARYAM ANAFCHEH ◽  
REZA GHAFOURI ◽  
FERESHTEH NADERI

DFT calculations are applied to evaluate the effects of atomic arrangements of dopant atoms on electronic features of the most stable structures of C 60−n N n(n = 1–12) fullerenes. Our study reveals that 13 C isotropic chemical shifts (δiso) of the nuclei at C–N pentagon–hexagon (ph) junctions appear at downfield values while there are no tangible values of δiso for the nuclei at C–N hexagon–hexagon (hh) junctions; the carbon sites attached to the first neighbors of nitrogen at hh junctions (the second neighboring effect) yield upfield values of δiso. Moreover, compensation between diatropic and paratropic ring currents leads to slightly less negative NICS value in C59N heterofullerene (-3.6) compared to the parent fullerene C60 (-4.25). However, with incorporating more nitrogen atoms into the cage the aromaticity first increases, up to the 66π-system C54N6 and C53N7 , yielding the most negative NICS values of -26.3 and -26.8, respectively, and then decreases so that NICS finally reaches the value of -6.6 ppm for C48N12 heterofullerene. On the basis of distinct value predicted for each heterofullerene, one expects that NICS values may also be useful for identification of the molecules through their endohedral 3 He NMR chemical shifts.


Chemistry ◽  
2021 ◽  
Vol 3 (3) ◽  
pp. 1005-1021
Author(s):  
Heike Fliegl ◽  
Maria Dimitrova ◽  
Raphael J. F. Berger ◽  
Dage Sundholm

A recently developed methodology for calculating, analyzing, and visualizing nuclear magnetic shielding densities is used for studying spatial contributions including ring-current contributions to 1H nuclear magnetic resonance (NMR) chemical shifts of aromatic and anti-aromatic free-base porphyrinoids. Our approach allows a visual inspection of the spatial origin of the positive (shielding) and negative (deshielding) contributions to the nuclear magnetic shielding constants. Diatropic and paratropic current-density fluxes yield both shielding and deshielding contributions implying that not merely the tropicity of the current density determines whether the contribution has a shielding or deshielding character. Instead the shielding or deshielding contribution is determined by the direction of the current-density flux with respect to the studied nucleus.


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