Rates of solvolysis of 2-methyl-, 2-tert-butyl-, 2-phenyl-, and 2-cyclopropyl-2-propyl p-nitrobenzoates. Evidence that carbon-13 chemical shifts of carbonium ions do not correlate with the stabilities of solvolytic transition states and the corresponding cationic intermediates

1973 ◽  
Vol 95 (7) ◽  
pp. 2400-2401 ◽  
Author(s):  
Herbert C. Brown ◽  
Edward N. Peters
Keyword(s):  
Chemical Shifts ◽  
Transition States ◽  
Tert Butyl ◽  
Carbonium Ions

scholarly journals Benchmarking the Fluxional Processes of Organometallic Piano-Stool Complexes

Molecules ◽  
2021 ◽  
Vol 26 (8) ◽  
pp. 2310
Author(s):  
Nathan C. Frey ◽  
Eric Van Dornshuld ◽  
Charles Edwin Webster
Keyword(s):  
Transition Metals ◽  
Density Functional ◽  
Chemical Shifts ◽  
Transition States ◽  
Electronic Energy ◽  
Basis Set ◽  
Composite Approach ◽  
Temperature Regimes ◽  
Correlation Consistent Composite Approach ◽  
Correlation Consistent

The correlation consistent Composite Approach for transition metals (ccCA-TM) and density functional theory (DFT) computations have been applied to investigate the fluxional mechanisms of cyclooctatetraene tricarbonyl chromium ((COT)Cr(CO)3) and 1,3,5,7-tetramethylcyclooctatetraene tricarbonyl chromium, molybdenum, and tungsten ((TMCOT)M(CO)3 (M = Cr, Mo, and W)) complexes. The geometries of (COT)Cr(CO)3 were fully characterized with the PBEPBE, PBE0, B3LYP, and B97-1 functionals with various basis set/ECP combinations, while all investigated (TMCOT)M(CO)3 complexes were fully characterized with the PBEPBE, PBE0, and B3LYP methods. The energetics of the fluxional dynamics of (COT)Cr(CO)3 were examined using the correlation consistent Composite Approach for transition metals (ccCA-TM) to provide reliable energy benchmarks for corresponding DFT results. The PBE0/BS1 results are in semiquantitative agreement with the ccCA-TM results. Various transition states were identified for the fluxional processes of (COT)Cr(CO)3. The PBEPBE/BS1 energetics indicate that the 1,2-shift is the lowest energy fluxional process, while the B3LYP/BS1 energetics (where BS1 = H, C, O: 6-31G(d′); M: mod-LANL2DZ(f)-ECP) indicate the 1,3-shift having a lower electronic energy of activation than the 1,2-shift by 2.9 kcal mol−1. Notably, PBE0/BS1 describes the (CO)3 rotation to be the lowest energy process, followed by the 1,3-shift. Six transition states have been identified in the fluxional processes of each of the (TMCOT)M(CO)3 complexes (except for (TMCOT)W(CO)3), two of which are 1,2-shift transition states. The lowest-energy fluxional process of each (TMCOT)M(CO)3 complex (computed with the PBE0 functional) has a ΔG‡ of 12.6, 12.8, and 13.2 kcal mol−1 for Cr, Mo, and W complexes, respectively. Good agreement was observed between the experimental and computed 1H-NMR and 13C-NMR chemical shifts for (TMCOT)Cr(CO)3 and (TMCOT)Mo(CO)3 at three different temperature regimes, with coalescence of chemically equivalent groups at higher temperatures.

Purine Nucleosides and Nucleotides Unequivocally in the Syn Conformation: Guanosine and 5′-GM P with 8-tert-Butyl and 8-( α-Hydroxyisopropyl) Substituents

1978 ◽  
Vol 33 (11-12) ◽  
pp. 902-907 ◽  
Author(s):  
Reynaldo Pless ◽  
Lech Dudycz ◽  
Ryszard Stolarski ◽  
David Shugar
Keyword(s):  
Alkaline Phosphatase ◽  
Chemical Shifts ◽  
Radical Reaction ◽  
Uv Spectra ◽  
Free Radical Reaction ◽  
Tert Butyl ◽  
Elementary Analysis ◽  
Analogous Manner ◽  
Van Der Waals Radii ◽  
Derivatives Of

Direct alkylation via a free radical reaction was utilized to prepare 8-tert-butylguanosine-5′- phosphate; this was dephosphorylated with alkaline phosphatase to give the parent nucleoside. The 8- (α-hydroxyisopropyl) derivatives of 5′-GMP and guanosine were prepared in an analogous manner. The foregoing products, which are necessarily in the syn conformation about the glycosidic bond because of the bulky 8-substituents (van der Waals’ radii 3.5-4 Å), were characterized by elementary analysis, UV spectra and 1HNMR spectroscopy. The changes in chemical shifts of H (1′), H (2′) and H (3′) of each of the foregoing derivatives, relative to those for guanosine and 5′-GMP, were consistent with their being in the syn conformation and, furthermore, pointed to guanosine and 5′-GMP being predominantly anti, in aqueous medium or DMSO. They also demonstrated that, contrary to the prevailing opinion, 8-bromo-5′-GMP and 8-bromoguanosine are not necessarily ex­clusively in the syn conformation in solution. While 8-bromoadenosine-5′-phosphate was slowly dephosphorylated by snake venom 5′-nucleoti- dase, the 8-tert-butyl and 8-(α-hydroxyisopropyl) derivatives of 5′-GMP were fully resistant to this enzyme.

THE NUCLEAR MAGNETIC RESONANCE SPECTRA OF TRIARYL CARBONIUM IONS

1959 ◽  
Vol 37 (9) ◽  
pp. 1402-1408 ◽  
Author(s):  
R. B. Moodie ◽  
T. M. Connor ◽  
Ross Stewart
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Trifluoroacetic Acid ◽  
Chemical Shifts ◽  
Trifluoroacetic Anhydride ◽  
Para Position ◽  
Single Peak ◽  
Carbonium Ions ◽  
Nuclear Magnetic Resonance Spectra ◽  
Neutral Compounds

The N.M.R. spectra of the triphenylmethyl cation and several of its derivatives substituted with methyl, methoxyl, and tert-butyl groups in the meta or para position have been obtained in trifluoroacetic acid – trifluoroacetic anhydride solution. The ring protons and substituent protons are all shifted to lower fields, i.e. are less shielded in the carbonium ions than in the neutral compounds of similar structure. The chemical shifts for the substituent protons are greater for para than for meta substituents and for mono- rather than tri-substituted ions. With each ion only a single peak was observed for the substituent protons but the ring protons were, in some cases, well resolved. The theoretical implications of these results are discussed.

Some Applications of Ultrahigh Resolution 15N NMR Spectroscopy

1998 ◽  
Vol 53 (4) ◽  
pp. 411-415 ◽  
Author(s):  
Bernd Wrackmeyer ◽  
Ēriks Kupče
Keyword(s):  
Nmr Spectroscopy ◽  
Nmr Spectra ◽  
Chemical Shifts ◽  
Absolute Magnitude ◽  
Natural Abundance ◽  
Coupling Constants ◽  
15N Nmr ◽  
Ultrahigh Resolution ◽  
Tert Butyl ◽  
15N Nmr Spectroscopy

Abstract Ultrahigh resolution 15N NMR spectra were measured for two nitroalkanes (MeNO2 la, tBuNO2 1c), two isocyanates (tBuNCO 2c, Me3SiNCO 2d), four isothiocyanates (MeNCS 3a, EtNCS 3b, tBuNCS 3c, Me3SiNCS 3d), one carbodiimide (Me3SiNCNSiMe3 4d), one keteneimine [Me3SiNCC(SiMe3)2 5d], two sulphinyl imides (tBuNSO 6c, Me3SiNSO 6d), and N-tert-butyl-pyrrole 7c, in order to determine coupling constants J(15N ,I3C) and isotope induced chemical shifts 1∆12/13C (15N) at the natural abundance of the isotopes. The values 1∆12/13C (15N) can be separated into two groups, one dealing with NC single and another one with NC double bonds. In each group (with few exceptions), the values 1∆12/13C (15N) become more negative with a decrease in the absolute magnitude of l1J(l5N ,13C)l. The corresponding values 1∆14/15N (13C) show a sim ilar behaviour. However, N-substituted pyrroles appear to be exceptional in this respect.

scholarly journals Probing the Fluxional Bonding Nature of Rapid Cope rearrangements in Bullvalene C10H10 and Its Analogs C8H8, C9H10, and C8BH9

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Yuan-Yuan Ma ◽  
Miao Yan ◽  
Hai-Ru Li ◽  
Yan-Bo Wu ◽  
Xin-Xin Tian ◽  
...  
Keyword(s):  
Model Compound ◽  
Chemical Shifts ◽  
Transition States ◽  
Ground States ◽  
H Nmr ◽  
Natural Bond Orbitals ◽  
Compound C ◽  
Shielding Tensors ◽  
Cope Rearrangements ◽  
C Nmr

AbstractBullvalene C10H10 and its analogs semibullvalene C8H8, barbaralane C9H10, and 9-Borabarbaralane C8BH9 are prototypical fluxional molecules with rapid Cope rearrangements at finite temperatures. Detailed bonding analyses performed in this work reveal the existence of two fluxional π-bonds (2 2c-2e π → 2 3c-2e π → 2 2c-2e π) and one fluxional σ-bond (1 2c-2e σ → 1 4c-2e σ → 1 2c-2e σ) in their ground states and transition states, unveiling the universal π + σ double fluxional bonding nature of these fluctuating cage-like species. The highest occupied natural bond orbitals (HONBOs) turn out to be typical fluxional bonds dominating the dynamics of the systems. The 13C-NMR and 1H-NMR shielding tensors and chemical shifts of the model compound C8BH9 are computationally predicted to facilitate future experiments.

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