Steric factors in the gas phase elimination kinetics of ethyl N‐benzyl‐N‐cyclopropylcarbamate and ethyl diphenylcarbamate

The elimination kinetics of ethyl N‐benzyl‐N‐cyclopropylcarbamate and ethyl diphenylcarbamate were investigated over the temperature range of 349.9–440.0°C and the pressure range of 31–106 Torr. These reactions have been found to be homogeneous, unimolecular, and obey a first‐order rate law. The products are ethylene, carbon monoxide, and the corresponding secondary amine. The rate coefficient is expressed by the following Arrhenius equations: For ethyl N‐benzyl‐N‐cyclopropylcarbamate log k₁ (s^?1) = (12.94 ± 0.09) ? (198.5 ± 0.9) kJ mol^?1 (2.303RT)^?1 For ethyl diphenylcarbamate log k₁ (s^?1) = (12.91 ± 0.18) ? (208.2 ± 2.4) kJ mol^?1 (2.303RT)^?1 The presence of phenyl and bulky groups at the nitrogen atom of the ethylcarbamate showed a decrease in the rate of elimination. Steric factor may be operating during the process of decomposition of these substrates. These reactions appear to undergo a semipolar six‐membered cyclic transition type of mechanism.© 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 67–71, 2002     

The elimination kinetics and mechanisms of ethyl piperidine‐3‐carboxylate,ethyl 1‐methylpiperidine‐3‐carboxylate,and ethyl 3‐(piperidin‐1‐yl)propionate in the gas phase

The gas‐phase elimination kinetics of the above‐mentioned compounds were determined in a static reaction system over the temperature range of 369–450.3°C and pressure range of 29–103.5 Torr. The reactions are homogeneous, unimolecular, and obey a first‐order rate law. The rate coefficients are given by the following Arrhenius expressions: ethyl 3‐(piperidin‐1‐yl) propionate, log k₁(s^?1) = (12.79 ± 0.16) ? (199.7 ± 2.0) kJ mol^?1 (2.303 RT)^?1; ethyl 1‐methylpiperidine‐3‐carboxylate, log k₁(s^?1) = (13.07 ± 0.12)–(212.8 ± 1.6) kJ mol^?1 (2.303 RT)^?1; ethyl piperidine‐3‐carboxylate, log k₁(s^?1) = (13.12 ± 0.13) ? (210.4 ± 1.7) kJ mol^?1 (2.303 RT)^?1; and 3‐piperidine carboxylic acid, log k₁(s^?1) = (14.24 ± 0.17) ? (234.4 ± 2.2) kJ mol^?1 (2.303 RT)^?1. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene through a concerted six‐membered cyclic transition state type of mechanism. The intermediate β‐amino acids decarboxylate as the α‐amino acids but in terms of a semipolar six‐membered cyclic transition state mechanism. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 106–114, 2006     

The gas‐phase elimination kinetics of ethyl 2‐furoate and ethyl 2‐thiophenecarboxylate

The gas‐phase elimination kinetics of ethyl 2‐furoate and 2‐ethyl 2‐thiophenecarboxylate was carried out in a static reaction system over the temperature range of 623.15–683.15 K (350–410°C) and pressure range of 30–113 Torr. The reactions proved to be homogeneous, unimolecular, and obey a first‐order rate law. The rate coefficients are expressed by the following Arrhenius equations: ethyl 2‐furoate, log k₁ (s^?1) = (11.51 ± 0.17)–(185.6 ± 2.2) kJ mol^?1 (2.303 RT)^?1; ethyl 2‐thiophenecarboxylate, log k₁ (s^?1) = (11.59 ± 0.19)–(183.8 ± 2.4) kJ mol^?1 (2.303 RT)^?1. The elimination products are ethylene and the corresponding heteroaromatic 2‐carboxylic acid. However, as the reaction temperature increases, the intermediate heteroaromatic carboxylic acid products slowly decarboxylate to give the corresponding heteroaromatic furan and thiophene, respectively. The mechanisms of these reactions are suggested and described. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 145–152, 2009     

Kinetics of the gas‐phase elimination of α‐Bromophenylacetic acid under maximum inhibition

The gas phase elimination kinetics of the title compound was studied over the temperature range of 260.1–315.0°C and pressure range of 20–70 Torr. This elimination, in seasoned static reaction system and in the presence of at least fourfold of the free radical inhibitor toluene, is homogeneous, unimolecular and follows a first‐order rate law. The reaction yielded mainly benzaldehyde, CO, and HBr, and small amounts of benzylbromide and CO₂. The observed rate coefficients are expressed by the following Arrhenius equations: For benzaldehyde formation: log k₁ (s⁻¹) = (12.23 ± 0.26) − (164.9 ± 2.7) kJ mol⁻¹ (2.303 RT)⁻¹ For benzylbromide formation: log k₁ (s⁻¹) = (13.82 ± 0.50) − (192.8 ± 5.5) kJ mol⁻¹ (2.303 RT)⁻¹ The mechanisms are believed to proceed through a semi‐polar five‐membered cyclic transition state for the benzaldehyde formation, while a four‐centered cyclic transition state for benzylbromide formation. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 725–728, 1999     

Kinetic and Mechanisms of the Homogeneous,Unimolecular Elimination of Phenyl Chloroformate and p‐Tolyl Chloroformate in the Gas Phase

The gas‐phase elimination of phenyl chloroformate gives chlorobenzene, 2‐chlorophenol, CO₂, and CO, whereasp‐tolyl chloroformate produces p‐chlorotoluene and 2‐chloro‐4‐methylphenol CO₂ and CO. The kinetic determination of phenyl chloroformate (440–480^oC, 60–110 Torr) and p‐tolyl chloroformate (430–480°C, 60–137 Torr) carried out in a deactivated static vessel, with the free radical inhibitor toluene always present, is homogeneous, unimolecular and follows a first‐order rate law. The rate coefficient is expressed by the following Arrhenius equations: Phenyl chloroformate: Formation of chlorobenzene, log k_I = (14.85 ± 0.38) – (260.4 ± 5.4) kJ mol^?1 (2.303RT)^?1; r = 0.9993 Formation of 2‐chlorophenol, log k_II = (12.76 ± 0.40) – (237.4 ± 5.6) kJ mol^?1(2.303RT)^?1; r = 0.9993 p‐Tolyl chloroformate: Formation of p‐chlorotoluene: log k_I = (14.35 ± 0.28) – (252.0 ± 1.5) kJ mol^–1 (2.303RT)^?1; r = 0.9993 Formation of 2‐chloro‐4‐methylphenol, log k_II = (12.81 ± 0.16) – (222.2 ± 0.9) kJ mol^?1(2.303RT)^–1; r = 0.9995 The estimation of the k_I values, which is the decarboxylation process in both substrates, suggests a mechanism involving an intramolecular nucleophilic displacement of the chlorine atom through a semipolar, concerted four‐membered cyclic transition state structure; whereas the k_II values, the decarbonylation in both substrates, imply an unusual migration of the chlorine atom to the aromatic ring through a semipolar, concerted five‐membered cyclic transition state type of mechanism. The bond polarization of the C–Cl, in the sense C^{δ+ …} Cl^δ?, appears to be the rate‐determining step of these elimination reactions.     

Linear correlation of polar substituents in the gas-phase pyrolyses of ethyl α-substituted acetates

The pyrolysis kinetics of several ethyl esters with polar substituents at the acyl carbon have been studied in the temperature range of 319.8–400.0°C and pressure range of 50.5–178.0 torr. These eliminations are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are given by the Arrhenius equations: for ethyl glycolate, log k₁ (s^?1) = (12.75 ± 0.30) – (201.4 ± 3.8) kJ/mol/2.303RT; for ethyl cyanoacetate, log k₁ (s^?1) = (12.19 ± 0.18) – (191.8 ± 2.1) kJ/mol/2.303RT; for ethyl dichloroacetate, log k₁ (s^?1) = (12.62 ± 0.36) – (193.9 ± 4.3) kJ/mol/2.303RT; for ethyl trichloroacetate, log k₁ (s^?1) = (12.27 ± 0.09) – (185.1 ± 1.0) kJ/mol/2.303RT. The results of the present work together with those reported recently in the literature give an approximate linear correlation when plotting log k/k₀ vs. σ* values (ρ* = 0.315 ± 0.004, r = 0.976, and intercept = 0.032 ± 0.006 at 400°C). This linear relationship indicates that the polar substituents affect the rate of elimination by electronic factors. The greater the electronegative nature of the polar substituent, the faster is the pyrolysis rate. The alkyl substituents yield, within experimental error, similar values in rates which makes difficult an adequate assessment of their real influence.     

Gas-phase elimination kinetics of 1-substituted ethyl acetates. Effect of polar substituents at the α carbon of secondary acetates

The gas-phase elimination of several polar substituents at the α carbon of ethyl acetates has been studied in a static system over the temperature range of 310–410°C and the pressure range of 39–313 torr. These reactions are homogeneous in both clean and seasoned vessels, follow a first-order rate law, and are unimolecular. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: 2-acetoxypropionitrile, log k₁ (s^?1) = (12.88 ± 0.29) – (203.3 ± 2.6) kJ/mol (2.303RT)^?1; for 3-acetoxy-2-butanone, log ±₁(s^?1) = (13.40 ± 0.20) – (202.8 ± 2.4) kJ/mol (2.303RT)^?1; for 1,1,1-trichloro-2-acetoxypropane, log ?₁ (s^?1) = (12.12 ± 0.50) – (193.7 ± 6.0) kJ/mol (2.303RT)^?; for methyl 2-acetoxypropionate, log ?₁ (s^?1) = (13.45 ± 0.05) – (209.5 ± 0.5) kJ/mol (2.303RT)^?1; for 1-chloro-2-acetoxypropane, log ?₁ (s^?1) = (12.95 ± 0.15) – (197.5 ± 1.8) kJ/mol (2.303RT)^?1; for 1-fluoro-2-acetoxypropane, log ?₁ (s^?1) = (12.83 ± 0.15)– (197.8 ± 1.8) kJ/mol (2.303RT)^?1; for 1-dimethylamino-2-acetoxypropane, log ?₁ (s^?1) = (12.66 ± 0.22) –(185.9 ± 2.5) kJ/mol (2.303RT)^?1; for 1-phenyl-2-acetoxypropane, log ?₁ (s^?1) = (12.53 ± 0.20) – (180.1 ± 2.3) kJ/mol (2.303RT)^?1; and for 1-phenyl?3?acetoxybutane, log ?₁ (s^?1) = (12.33 ± 0.25) – (179.8 ± 2.9) kJ/mol (2.303RT)^?1. The C_α? O bond polarization appears to be the rate-determining process in the transmition state of these pyrolysis reactions. Linear correlations of electron-releasing and electron-withdrawing groups along strong σ bonds have been projected and discussed. The present work may provide a general view on the effect of alkyl and polar substituents at the C_α? O bond in the gas-phase elimination of secondary acetates.     

The retro-aldol mechanism in the pyrolysis kinetics of primary,secondary, and tertiary β-hydroxy ketones in the gas phase

The pyrolysis kinetics of primary, secondary, and tertiary β-hydroxy ketones have been studied in static seasoned vessels over the pressure range of 21–152 torr and the temperature range of 190°–260°C. These eliminations are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are expressed by the following equations: for 1-hydroxy-3-butanone, log k₁(s^?1) = (12.18 ± 0.39) ? (150.0 ± 3.9) kJ mol^?1 (2.303RT)^?1; for 4-hydroxy-2-pentanone, log k₁(s^?1) = (11.64 ± 0.28) ? (142.1 ± 2.7) kJ mol^?1 (2.303RT)^?1; and for 4-hydroxy-4-methyl-2-pentanone, log k₁(s^?1) = (11.36 ± 0.52) ? (133.4 ± 4.9) kJ mol^?1 (2.303RT)^?1. The acid nature of the hydroxyl hydrogen is not determinant in rate enhancement, but important in assistance during elimination. However, methyl substitution at the hydroxyl carbon causes a small but significant increase in rates and, thus, appears to be the limiting factor in a retroaldol type of mechanism in these decompositions. © John Wiley & Sons, Inc.     

The kinetics and mechanisms of gas phase elimination of primary,secondary, and tertiary 2‐hydroxyalkylbenzenes

2‐Phenylethanol, racemic 1‐phenyl‐2‐propanol, and 2‐methyl‐1‐phenyl‐2‐propanol have been pyrolyzed in a static system over the temperature range 449.3–490.6°C and pressure range 65–198 torr. The decomposition reactions of these alcohols in seasoned vessels are homogeneous, unimolecular, and follow a first‐order rate law. The Arrhenius equations for the overall decomposition and partial rates of products formation were found as follows: for 2‐phenylethanol, overall rate log k₁(s⁻¹)=12.43−228.1 kJ mol⁻¹ (2.303 RT)⁻¹, toluene formation log k₁(s⁻¹)=12.97−249.2 kJ mol⁻¹ (2.303 RT)⁻¹, styrene formation log k₁(s⁻¹)=12.40−229.2 kJ mol⁻¹(2.303 RT)⁻¹, ethylbenzene formation log k₁(s⁻¹)=12.96−253.2 kJ mol⁻¹(2.303 RT)⁻¹; for 1‐phenyl‐2‐propanol, overall rate log k₁(s⁻¹)=13.03−233.5 kJ mol⁻¹(2.303 RT)⁻¹, toluene formation log k₁(s⁻¹)=13.04−240.1 kJ mol⁻¹(2.303 RT)⁻¹, unsaturated hydrocarbons+indene formation log k₁(s⁻¹)=12.19−224.3 kJ mol⁻¹(2.303 RT)⁻¹; for 2‐methyl‐1‐phenyl‐2‐propanol, overall rate log k₁(s⁻¹)=12.68−222.1 kJ mol⁻¹(2.303 RT)⁻¹, toluene formation log k₁(s⁻¹)=12.65−222.9 kJ mol⁻¹(2.303 RT)⁻¹, phenylpropenes formation log k₁(s⁻¹)=12.27−226.2 kJ mol⁻¹(2.303 RT)⁻¹. The overall decomposition rates of the 2‐hydroxyalkylbenzenes show a small but significant increase from primary to tertiary alcohol reactant. Two competitive eliminations are shown by each of the substrates: the dehydration process tends to decrease in relative importance from the primary to the tertiary alcohol substrate, while toluene formation increases. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 401–407, 1999     

The pyrolysis kinetics of ethyl esters in the gas phase. The alkyl substituent effect at the acyl carbon

The gas-phase elimination of ethyl 3-methylbutanoate and ethyl 3,3-dimethylbutanoate has been studied, in a static system, over the temperature range of 360–420°C and in the pressure range of 71–286 torr. The reactions are homogeneous, unimolecular, and follow a first-order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for ethyl 3-methylbutanoate, log k₁ (s^?1) = (12.70 ± 0.36) – (202.5 ± 4.4) kJ/mol/2.303RT, and for ethyl 3,3-dimethylbutanoate, log k₁ (s^?1) = (13.04 ± 0.08) – (207.1 ± 1.0) kJ/mol/2.303RT. Alkyl substituents at the acyl carbon of ethyl esters yield very close values in rates. Consequently it is rather difficult to offer some conclusion concerning the effect of these substituents.     

The mechanisms of the homogeneus,unimolecular elimination kinetics of ethyl 4-chlorobutyrate and 4-chlorobutyric acid in the gas phase

Ethyl 4-chlorobutyrate, which is reexamined, pyrolyzes at 350–410°C to ethylene, butyrolactone, and HCl. Under the reaction conditions, the primary product 4-chlorobutyric acid is responsible for the formation of γ-butyrolactone and HCl. In seasoned vessels, and in the presence of a free-radical inhibitor, the ester elimination is homogeneous, unimolecular, and follows a first-order rate law. For initial pressures from 69–147 Torr, the rate is given by the following Arrhenius expression: log k₁(s^?1) = (12.21 ± 0.26) ? (197.6 ± 3.3) kJ mol^?1 (2.303RT)^?1. The rates and product formation differ from the previous work on the chloroester pyrolysis. 4-Chlorobutyric acid, an intermediate product of the above substrate, was also pyrolyzed at 279–330°C with initial pressure within the range of 78–187 Torr. This reaction, which yields γ-butyrolactone and HCl, is also homogeneous, unimolecular, and obeys a first-order rate law. The rate coefficient, is given by the following Arrhenius equation: log k₁(s^?1) = (12.28 ± 0.41) ? (172.0 ± 4.6) kJ mol^?1 (2.303RT)^?1. The pyrolysis of ethyl chlorobutyrate proceeds by the normal mechanism of ester elimination. However, the intermediate 4-chlorobutyric acid was found to yield butyrolactone through anchimeric assistance of the COOH group and by an intimate ion pair-type of mechanism. Additional evidence of cyclic product and neighboring group participation is described and presented.     

The electronic effects of substituents at the acyl carbon in the gas-phase elimination kinetics of tert-butyl α-substituted acetates

The gas-phase eliminations of several tert-butyl esters, in a static system and in vessels seasoned with allyl bromide, have been studied in the temperature range of 171.5–280.1°C and the pressure range of 23–98 torr. The rate coefficients for the homogeneous unimolecular elimination of these esters are given by the following Arrhenius equations: for tert-butyl pivalate, log k₁(s^?1) = (13.44 ± 0.30) ? (169.1 ± 3.1) kJ · mol^?1 (2.303RT)^?1; for tert-butyl trichloroacetate, log k₁(s^?1) = (12.41 ± 0.08) ? (141.1 ± 0.7) kJ · mol^?1 (2.303RT)^?1; and for tert-butyl cyanoacetate log k₁(s^?1) = (11.31 ± 0.44) ? (137.8 ± 4.1) kJ · mol^?1 (2.303RT)^?1. The data of this work together with those reported in the literature yield a good linear relationship when plotting log k/k₀ vs. σ* values (ρ* = 0.635, correlation coefficient r = 0.972, and intercept = 0.048 at 250°C). The positive ρ* value suggests that the movement of negative charge to the acyl carbon in the transition state is rate determining. The present results along with previous investigations ratify the generalization that electron-withdrawing substituents at the acyl side of ethyl, isopropyl, and tert-butyl esters enhance the elimination rates, while electron-releasing groups tend to reduce them. The negative nature of the acyl carbon and the polarity in the transition state increases slightly from primary to tertiary esters.     

Kinetic Determination of the Gas‐Phase Decarbonylation of Butyraldehyde in the Presence of HCl Catalyst

The gas‐phase kinetics and mechanism of the homogeneous elimination of CO from butyraldehyde in the presence of HCl has been experimentally studied. The reaction is homogeneous and follows the second‐order kinetics with the following rate expression: log k ₁ (s⁻¹ L mol⁻¹) = (13.27 ± 0.36) – (173.2 ± 4.4) kJ mol⁻¹(2.303RT )⁻¹. Experimental data suggested a concerted four‐membered cyclic transition state type of mechanism. The first and rate‐determining step occurs through a four‐membered cyclic transition state to produce propane and formyl chloride. The formyl chloride intermediate rapidly decomposes to CO and HCl gases.     

Experimental and theoretical study of the homogeneous,unimolecular gas‐phase elimination kinetics of 2‐furoic acid

The kinetics of the gas‐phase elimination kinetics of CO₂ from furoic acid was determined in a static system over the temperature range 415–455°C and pressure range 20–50 Torr. The products are furan and carbon dioxide. The reaction, which is carried out in vessels seasoned with allyl bromide and in the presence of the free‐radical suppressor toluene and/or propene, is homogeneous, unimolecular, and follows a first‐order rate law. The observed rate coefficient is expressed by the following Arrhenius equation: log k₁(s^?1) = (13.28 ± 0.16) ? (220.5 ± 2.1) kJ mol^?1 (2.303 RT)^?1. Theoretical studies carried out at the B3LYP/6‐31++G** computational level suggest two possible mechanisms according to the kinetics and thermodynamic parameters calculated compared with experimental values. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 298–306, 2007     

Kinetics of thermal gas‐phase decomposition of 2‐bromopropene using static system

The gas phase elimination kinetics of 2‐bromopropene was studied over the temperature range of 571–654 K and pressure range of 12–46 Torr using the seasoned static reaction system. Propyne was the only olefinic product formed and accounted for >98% of the reaction. This product was formed by homogeneous, unimolecular pathways with high‐pressure first‐order rate constant k_∞ given by the equation k_∞ = 10^{13.47 ± 0.6} exp^{?208.2 ± 6.7} (kJ mol^?1)/RT. The error limits are 95% certainty limits. The observed Arrhenius parameters are consistent with the four centered activated complex. The presence of methyl group on α‐carbon lowers the activation energy by 41 kJ mol^?1. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 1–5, 2007     

The elimination kinetics of secondary Alkyl Bromides in the gas phase

Elimination kinetics of 2-bromohexane and 2-bromo-4-methylpentane in the gas phase were examined over the temperature range of 310–360°C and pressure range of 46–213 torr. The reactionsin seasoned, static reaction vessels, and in the presence of the free radical inhibitor cyclohexene, are homogeneous, unimolecular, and follow first order rate laws. The overall rate coefficients are described by the following Arrhenius equations: For 2-bromohexane, log??₁(s^?1) = (13.08 ± 0.70) ? (185.7 ± 8.2) kJ mol^?1 (2.303RT)^?1; for 2-bromo-4-methylpentane, log??₁(s^?1) = (13.08 ± 0.33) ? (183.4 ± 3.8) kJ mol^?1 (2.303RT)^?1. The electron releasing effect of alkyl groups influences the overall elimination rates. The olefin products isomerize in the presence of HBr gas until an equilibrium mixture is reached.     

The gas-phase elimination kinetics of 3-buten-1-methanesulphonate and 3-methyl-3-buten-1-methanesulphonate

The elimination kinetics of the title compounds were carried out in a static system over the temperature range of 290–330°C and pressure range of 29.5–124 torr. The reactions, carried out in seasoned vessels with allyl bromide, obey first-order rate law, are homogeneous and unimolecular. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for 3-buten-1-methanesulphonate, log k₁(s^?1) = (12.95 ± 0.53) ? (175.3 ± 5.9)kJ mol^?1(2.303RT)^?1; and for 3-methyl-3-buten-1-methanesulphonate, log k₁(s^?1) = (12.98 ± 0.40) ? (174.7 ± 4.5)kJ mol^?1(2.303RT)^?1. The olefinic double bond appears to assist in the rate of pyrolysis. The mechanism is described in terms of an intimate ion-pair intermediate. © 1995 John Wiley & Sons, Inc.     

The kinetics and mechanisms of methyl 2-bromopropionate and 2-bromopropionic acid pyrolyses under maximum inhibition

The kinetics of the gas phase pyrolyses of methyl 2-bromopropionate and 2-bromopropionic acid were studied in a seasoned, static reaction vessel and under maximum inhibition of the free radical suppressor toluene. The working temperature and pressure range was 310–430°C and 26.5–201.5 torr, respectively. The reactions proved to be homogeneous, unimolecular, and obeys a first-order rate law. The rate coefficients are expressible by the following equations: for methyl 2-bromopropionate, log k₁(s^?1) = (13.10 ± 0.34) ? (211.4 ± 4.4)kJ mol^?1(2.303RT)^?1; for 2-bromopropionic acid, log k₁(s^?1) = (12.41 ± 0.29) ? (180.3 ± 3.4)kJ mol^?1(2.303RT)^?1. The bromoacid yields acetaldehyde, CO and HBr. Because of this result, the mechanism is believed to proceed via a polar five-membered cyclic transition state.     

Ligand Dynamics in the Solid: Lithium‐fluorenide and Lithium‐benzo[b]fluorenide N,N,N′,N′‐Tetramethylethylenediamine (tmeda) Complexes

The dynamic behavior of the N,N,N′,N′‐tetramethylethylenediamine (tmeda) ligand has been studied in solid lithium‐fluorenide(tmeda) ( 3 ) and lithium‐benzo[b]fluorenide(tmeda) ( 4 ) using CP/MAS solid‐state ¹³C‐ and ¹⁵N‐NMR spectroscopy. It is shown that, in the ground state, the tmeda ligand is oriented parallel to the long molecular axis of the fluorenide and benzo[b]fluorenide systems. At low temperature (<250 K), the ¹³C‐NMR spectrum exhibits two MeN signals. A dynamic process, assigned to a 180° rotation of the five‐membered metallacycle (π‐flip), leads at elevated temperatures to coalescence of these signals. Line‐shape calculations yield ΔH^?=42.7 kJ mol^?1, ΔS^?=?5.3 J mol^?1 K^?1, and =44.3 kJ mol^?1 for 3 , and ΔH^?=36.8 kJ mol^?1, ΔS^?=?17.7 J mol^?1 K^?1, and =42.1 kJ mol^?1 for 4 , respectively. A second dynamic process, assigned to ring inversion of the tmeda ligand, was detected from the temperature dependence of T_1ρ, the ¹³C spin‐lattice relaxation time in the rotating frame, and led to ΔH^?=24.8 kJ mol^?1, ΔS^?=?49.2 J mol^?1 K^?1, and =39.5 kJ mol^?1 for 3 , and ΔH^?=18.2 kJ mol^?1, ΔS^?=?65.3 J mol^?1 K^?1, and =37.7 kJ mol^?1 for 4 , respectively. For (D₁₂)‐ 3 , the rotation of the CD₃ groups has also been studied, and a barrier E_a of 14.1 kJ mol^?1 was found.     

Differentiation of α‐COOH from β‐COOH in aspartic acids by N‐phosphorylation

The biomimic reactions of N‐phosphoryl amino acids, which involved intramolecular penta‐coordinate phosphoric‐carboxylic mixed anhydrides, are very important in the study of many biochemical processes. The reactivity difference between the α‐COOH group and β‐COOH in phosphoryl amino acids was studied by experiments and theoretical calculations. It was found that the α‐COOH group, and not β‐COOH, was involved in the ester exchange on phosphorus in experiment. From MNDO calculations, the energy of the penta‐coordinate phosphoric intermediate containing five‐member ring from α‐COOH was 35 kJ/mol lower than that of the six‐member one from β‐COOH. This result was in agreement with that predicted by HF/6‐31G** and B3LYP/6‐31G** calculations. Theoretical three‐dimensional potential energy surface for the intermediates predicted that the transition states 4 and 5 involving α‐COOH or β‐COOH group had energy barriers of ΔE=175.8 kJ?mol^?1 and 210.4 kJ?mol^?1, respectively. So the α‐COOH could be differentiated from β‐COOH intramolecularly in aspartic acids by N‐phosphorylation. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 41–51, 2001