bluuebunny/crossref_metadata_2025_split huggingface dataset:
Untersuchungen zum Einfluss der Stereochemie auf die Elektronenstossinduzierte ‘Retro‐Diels‐Alder’‐Reaktion
Authors: Bohlmann Ferdinand , Fischer Christian‐Herbert , Förster Jürgen , Mather Wolfgang , Schwarz Helmut 
Published: May 2005

AbstractDurch die Untersuchung geeigneter Modellverbindungen wird belgt, daß eine Generalisierung der Mechanismen der elektronenstoßinduzierten ‘Retro‐Diels‐Alder’‐Reaktion aufgrund sterischer Effekte unmöglich ist.
An intermolecular methyl transfer during field desorption mass spectrometry. Identification of the [M + 14] peak in choline chloride
Authors: Wood Gordon W. , Lau P.‐Y. 
Published: May 2005

AbstractCholine chloride, bromide and iodide all contain a peak at m/e 118 [M + 14] in their field desorption mass spectra. Comparison with choline(methyl‐d9) chloride demonstrates that this peak arises from an intermolecular transfer of a methyl group. Partial exchange of hydroxyl hydrogen on choline chloride does not displace the m/e 118 peak, suggesting that this hydrogen is lost during methyl transfer. The structure of the [M + 14] species should then correspond to choline methyl ether.
Analytical ion cyclotron resonance spectrometry. Spectereochemical factors and a functional group interaction in some norbornyl systems
Authors: Kao Jar‐Lin , Simonton Charles A. , Bursey Maurice M. 
Published: May 2005

AbstractIon cyclotron resonance results show that the rate of the acetylation of endo‐bicyclo‐[2.2.1]hept‐5‐en‐2‐ol by [(CH3CO)3]+ in the gas phase is similar to that of its exo‐isomer and greater than that of endo‐norborneol by the same ion. This suggests that steric interference of the 6‐endo hydrogen is removed by making it vinylic. Additionally, the acetates of the isomeric norborneols and of exo‐ and endo‐bicyclo[2.2.1]hept‐5‐en‐2‐ol have relative rates 1.0, 0.9, 1.1 and 0.5; these figures may be interpreted to indicate that acetylation may occur on the carbonyl oxygen here and that in the last compound the carbonyl function is protected by interaction with the olefinic bond.
The mass spectra of some chromans
Authors: Middleditch Brian S. , Macnicol David D. 
Published: May 2005

AbstractThe mass spectra of 4‐p‐hydroxyphenyl‐2,2,4‐trimethylchroman and of its thia, selena and sulphonyl analogues are reported and discussed. Formal charge localization on the oxygen atom of the oxo compound is reflected in the production of an abundant [M ‐ 15]+ ion. Lack of charge location on the heteroatom of the seleno compound leads to a retro‐Diels‐Alder reaction. The thio compound exhibits intermediate behaviour, but charge localization on the oxygen atoms is preferred in the sulphone.
Mass spectrometry of organic compounds of the group V elements. IV—new data on amines, phosphines, arsines and the corresponding acyl derivatives
Authors: Kostyanovsky R. G. , Pleshkova A. P. , Voznesensky V. N. , Elnatanov Yu. I. 
Published: May 2005

AbstractFurther investigations have given new data confirming differences in fragmentation under electrol impact of compounds of trivalent nitrogen and other group V elements. These data have been obtained by comparision of the fragmentation of the amines, phosphines, arsines, benzyl derivatives, geminal systems and acyl derivatives. Mass spectra of tetraethyl derivatives of the group IV elements and acetyl triethyl germane have also been taken and compared with the previous ones.
Chain length effects in the competition between intramolecular hydrogen migrations in the molecular ions of hydrogen migration in the molecular ions of α‐benzylamino‐ω‐benzyloxyalkanes as studied by deuterium labelling
Authors: Bruins Andries P. , Nibbering Nico M. M. 
Published: May 2005

AbstractThe unimolecular decompositions upon electron impact of the bifunctional compounds ØCH2‐NH(CH2)n‐OCH2Ø(n=2 to 7)have been studied by deuterium labelling. The molecular ions lose a benzyl radical and/or a benzaldehyde molecule, depending on the length of the aliphatic chain. Both reactions appear to be the result of the transfer of a hydrogen atom to the charge localized amino function in a cyclic transition state. The ring size of this transition state turns out to play a far more important role than the difference in reactivity of the migrating hydrogen atoms.
Mass spectra of some thioamide vinylogues and of their thiopyran derivatives
Authors: Pradère Jean‐Paul , Duguay Guy , Quiniou Hervé 
Published: May 2005

AbstractThe analysis of metastable ions which decompose in the second field free region of a reversed geometry double focusing mass spectrometer is used to study the fragmentation of a series of thioamide vinylogues and their thiopyran derivatives. The fragmentation processes are discussed and the expected occurrence of an electron impact‐induced retro‐Diels‐Alder reaction is confirmed.
Chelation reactions of acetylacetone on a field ion emitter
Authors: Röllgen F. W. , Schulten H.‐R. 
Published: May 2005

AbstractReactions of metal chlorides adsorbed on a field ion emitter with acetylacetone admitted from the gas phase lead to the formation of a number of different acetylacetonate ions. It is shown that these ions originate mainly from chelation reactions in which acetylacetonate molecules are formed prior to field ionization.
Mass spectrometric investigation of ethyl‐containing siloxanes and aromatic silyl ethers
Authors: Mysov E. I. , Akhrem I. S. , Avetisyan D. V. 
Published: May 2005

AbstractThe mass spectra of four linear ethylsiloxanes and eight aromatic silyl ethers are reported. General fragmentation patterns for the benzyltriethylsilyl ethers and their vinylogues are discussed. The spectra of the ethylsiloxanes and ethers are compared with those of their methyl analogues.
Spectres de masse de P‐anisyl‐6 2H‐Thiopyrannes Substitues ou non en 2 et Comportant en 3 un Groupement Fonctionnel
Authors: Pradère Jean‐Paul , Quiniou Hervé 
Published: May 2005

AbstractLes p‐anisyl‐6 2H‐thiopyrannes substitués en 3 par les groupements fonctionnels R3 = CHO, COCH3, CONH2, et CONCHN(CH3)2 ont des pics de base [M ‐ R3]+ (cationsthiopyrylium) correspondant à l'élimination du groupement fonctionnel. Lorsque R2 = OC2h5, l'élimination de R2 fournit d'autres cations thiopyrylium. Avec R2 = OC2h5 et R3 = COCH3, l'élimination de C2H6 fournit l'acétyl‐3 2H‐tyiopyrannone‐2 substituée contractée ultérieurement en dérivé thiophénique. Lorsque R3 ‐ SCHN2 ou CSN=SHN(CH3)2, les pics moléculaires sont les pics de base; dans cette série, fragmentations corespondant à l'elimination de H2S ou de HCSN (CH3)2 convertissent R3 en CN; dans la mémem série el la Série et la série oxygénée homologue, des fragmentations correspondent à l'élimination de NH2 ou de C3H7N2 portant sur R3.
Massenspektroskopische Fragmentierungsreaktionen. XI‐,β‐Spaltung von 2‐Alkylpyridinen‐Umlagerung zu einer Azachinodimethanstruktur?
Authors: Budzikiewicz H. , Besler U. 
Published: May 2005

AbstractDeuterium and 13C labelling shows that the β‐cleavage product of 2‐alkyl pyridines loses inter alia in the next step the 2′‐methylene group together with the ring nitrogen.
Pilzpigmente. XXV—Massenspektrometrische Untersuchung Polyisoprenoider Ansabenzochinone
Authors: Schwarz Helmut , Pasupathy Vinayagar , Steglich Wolfgang 
Published: May 2005

AbstractDie Massenspektren von Tridentochinon und seinen Derivaten sind charakteristisch verschieden von denen der biogentisch verwandten Bovichinone. Ein Teil des komplexen Fragmentierungsverhaltens konnte durch hochauflösende Massenspektrometrie, Verwendung der Defokussierungstechnik und im Fall des Secotridentochinons durch Untersuchung der [OD]‐ markierten Verbindung aufgeklärt werden.
The electron impact‐induced fragmentation of perchlorylbenzene
Authors: Giumanini Angelo G. , Lunelli Bruno 
Published: May 2005

AbstractThe modes of fragmentation of perchlorylbenzene (C6H5ClO3) and its perdeutero derivative under electron impact are reported and discussed. The occurrence of the [C6H5O]− and [C6H5O]+ ions is accounted for in terms of a novel phenyl migration from chlorine to oxygen.
A comparison of negative and positive ion mass spectrometry
Authors: Large Robert , Knof Hans 
Published: May 2005

AbstractNegative ion mass spectrometry using a conventional mass spectrometer with a special ion source and a sample pressure of approximately 2 × 10−5 Torr is shown to be an excellent method for the qualitative analysis of lower mass alcohols, mercaptans, ketones, aldehydes, aliphatic carboxylic acids and esters, the spectra of which are characterized by intense [M – H]− ions. The method may be termed a ‘selective’ low energy ionization technique, being suitable for the above organic compounds, but not for nitriles, nitro compounds, hydrocarbons, ethers, amines, amides, nitrogen heterocycles and chlorinated compounds. This method can be looked upon as a complementary method, to positive ion mass spectrometry.
The loss of water from isotopically labelled heptanals on chemical ionization
Authors: Fales Henry M. , Fenselau Catherine , Duncan James H. 
Published: May 2005

AbstractThe chemical ionization spectra of a series of deuterated heptanals has been studied using methane and isobutane. Loss of water from the [MH]+ ion follows the same pattern as in electron ionization; hydrogen (deuterium) is lost from all positions except C‐1 and C‐2. The reagent proton is found in the water lost only to the extent of 28% and this is confirmed using deuteriomethane. Possible mechanisms are discussed.
Note on the application of low voltage mass spectrometry to stereochemical problems
Authors: Zaikin V. G. , Smetanin V. I. , Wulfson N. S. 
Published: May 2005

AbstractUsing the stereoisomeric 1,2‐dimethyl‐4‐alkyldecahydroquinol‐4‐ols and various examples described elsewhere it was shown that there are many cases when decreasing of the electron energy does not lead to an increase in the quantitative differences between the mass spectra of stereoisomers.
Rearrangements 1,3 en Serie Heterocyclique. VI—Fragmentation, en Spectrometrie de Masse, des Aryl‐2 Alkyl‐4 Oxadiazol‐1,3,4 Ones‐5 et des Aryl‐2 Alcoxy‐5 Oxadiazoles‐1,3,4
Authors: Golfier Michel , Milcent René , Bouchoux Guy , Henry Yannik 
Published: May 2005

AbstractLes spectres de masse des aryl‐2 alkyl‐4 oxadiazol‐1,3,4 ones‐5 et de leurs isoméres aryl‐2 alcoxy‐5 oxadiazoles‐1,3,4 ont été étudiés. La fragmentation de ces composés ne semble pas faire intervenir d'isomérisation unimoléculaire d'origine électronique. Pour chaque type d'hétérocycle, les transferts d'hydrogéne accompagnant la pete de la chaîne alcoylée sont principalement induits par l'azote 4.
Etude en Serie as‐Triazine. XIX—Fragmentations Principales de s‐Triazolo‐as‐Triazinones en Spectrometrie de Masse
Authors: Daunis Jacques , Follet Michel 
Published: May 2005

AbstractLes fragmentations principales de s‐triazolo[4,3‐b]‐, [3,4‐c]‐ et ‐[3,2‐c]‐as triazinones ainsi que celles de leurs dérivés N‐méthylés ont été élucidées grâe á l'étude des spectres de masse des différents composés mono, di‐et triméthylés. Dans tous les cas nous avons observé en premier lieu uniquement la dèradation du cycle triazine qui expulse CO, RCN ou RCN CO selon la structure isomére des triazolo‐triazinones. La comparaison des specres de ces composés avec ceux de leurs dérivés N‐méthylés de structure fixe permet de préciser la participation de chaque forme á l'équilibre tautomére.
On the formation and decomposition of the [M  HNO]+ ion from o‐nitrobenzaldoxime
Authors: Vijfhuizen Peter C. , Heerma Wigger , Nibbering Nico M. M. 
Published: May 2005

AbstractThe molecular ion of o‐nitrobenzaldoxime appears to eliminate a molecule of HNO, the latter as shown by deuterium labelling containing the hydroxyl hydrogen atom. This reaction may be followed either by loss of CO2 or C2O2 (2 × CO), which requires complicated skeletal rearrangements. To determine the structure of the [M  HNO]+ ion, the kinetic energies, released in the loss of CO2 and of C2O2 from this ion, have been measured. Similar measurements have been made on the appropriate metastable ions from o‐nitrosobenzaldehyde, 2,1‐benzisoxazoline‐3‐one, 2‐benzoxazolinone and 3‐hydroxy‐1,2‐benzisoxazole, whose molecular ions have the same elemental composition as the [M  HNO]+ ion from o‐nitrobenzaldoxime.In the loss of HNO from the molecular ions of o‐nitrobenzaldoxime o‐nitrosobenzaldehyde molecular ions are generated first, which then, at least upon decomposition, rearrange further to 2,1‐benzisoxazoline‐3‐one molecular ions. Part of the latter finally rearrange to 2‐benzoxazolinone molecular ions, which eliminate CO via two routes. One of these corresponds to a low energy process and the other to a higher energy path whose excess energy induces the subsequent rapid loss of a second CO molecule. The observed effect of the rate‐determining isomerizations of the molecular ions of o‐nitrobenzaldoxime, o‐nitrosobenzaldehyde, 2,1‐benzisoxazoline‐3‐one and 3‐hydroxy‐1,2‐benzisoxazole to 2‐benzoxazolinone on the intensity and shape of the metastable peak for the low energy process for CO expulsion from the molecular ions of 2‐benzoxazolinone supports the proposed rearrangements.
Etude en Spectrometrie de Masse. VIII—Fragmentation de Qulques Benzalacetophenones et Benzalacetones Diversement Substituees sur le Cycle Aromatique
Authors: Rouvier Emile , Medina Haïm , Cambon Aimé 
Published: May 2005

AbstractNous présentons dans ce travail und étude comparative en spectrométrie de masse, du comportement des diphényl‐1,3 propénones (ou benazlacétophénones) et des phényl‐4 butanones‐2 (ou benazlacétones) dieversement substituées. La grande conjugaison qui s'étend á la quasi totalité de Ia molécule en fait des composés trés stables sous l'impact électronique. La présence d'un groupemen sur le cycle aromatique donne des fragements caratéristiques dont nous avons pu apprécier l'importance.
Intramolecular hydrogen exchanges during the electron impact‐induced fragmentation of complex alicyclic amines
Authors: Longevialle Pierre , Marazano Christian 
Published: May 2005

AbstractThe three‐step mechanism of formation of the important fragment ion of mass 84 from 3‐dimethylaminosteroids under electron impact is accompanied by intramolecular hydrogen exchanges, as shown by the mass spectra of deuterium labelled derivatives. The hydrogen exchanges are shown to occur in the ring A opened molecular ion, i.e. after the initial C‐3–‐C‐4 bond rupture, and to compete with the final C‐1–‐C‐10 bond rupture. (The intensity of hydrogen‐deuterium exchange reactions depends upon the activation energy of the final rupture.) The electron impact‐induced‐fragmentation of large alicyclic molecules bearing a strong charge stabilizing functional group, such as amines, appears to be the result of a series of displacements of the radical over different sites of the molecular ion through hydrogen (or carbon) rearrangements, with some of the radical sites being in the appropriate position to induce a final rupture.
A study of the electron impact‐induced fragmentation of 1,6‐diaminohexane
Authors: Blac Pierre‐Alain , Singy Gérard A. , Buchs Armand 
Published: May 2005

AbstractThe mass spectrometric behaviour of 1,6‐diaminohexane has been studied with the help of deuterium labelling in every position of the molecule. Extensive intramolecular exchange reactions between the hydrogen atoms of the amino groups and those of the hydrocarbon chain are responsible for the apparent complexity of the fragmentation mode.
Mass spectra of sulfinamide derivatives and identification of their thermal decomposition products
Authors: Ueda Yo , Mori Kōichi , Kouno Keiki , Nhya Tokihiro 
Published: May 2005

AbstractThe mass spectra of 30 sulfinamide derivatives (RSONHR', R' alkyl or p‐XC6H4) are reported. Most of the spectra had peaks attributable to thermal decomposition products. For some compounds these were identified by pyrolysis under similar conditions to be: RSO2NHR', RSO2SR, RSSR and NH2R' (in all kinds of sulfinyl amides); RSNHR' (in the case of arylsulfinyl arylamides); RSO2C6H4NH2, RSOC6H4NH2 and RSC6H4NH2 (in the case of arylsulfinyl arylamides of the type of X = H)The mass spectra of the three thermally stable compounds showed that there are several kinds of common fragment ions. The mass spectra of the thermally labile compounds had two groups of ions; (i) characteristic fragment ions of the intact molecules and (ii) the molecular ions of the thermal decomposition products. It was concluded that the sulfinamides give the following ions after electron impact: [M]+, [M  R]+, [M  R + H]+, [M  SO]+, [RS]+, [NHR']+, [NHR' + H]+, [RSO]+, [RSO + H]+, [R]+, [R + H]+, [R']+ and [M  OH]+, and that the thermal decomposition products give the following ions: [RSO2SR]+, [RSSR]+, [M  O]+, [M + O]+ and [RSOC6H4NH2]+.
Hydrogen migrations in mass spectrometry. II—single and double hydrogen migrations in the electron impact fragmentation of n‐propyl benzoate
Authors: Benoit Frank M. , Harrison Alex G. 
Published: May 2005

AbstractThe sources of the migrant hydrogen atom(s) in reactions (a) and (b) in the electron impact mass spectrum of n‐propyl benzoate have been investigated: (a) [C6H5CO2C3H7]+ →[C6H5CO2H]+ + C3H6; (b) [C6H5CO2C3H7]+ → [C6H5CO2H2]+ + C3H5sdot;. Deuterium labelling of the propyl group showed that, for reaction (a) at 70 eV ionizing energy 3 ± 1% of the hydrogen originates from C‐1 of the propyl group, 86 ± 4% from C‐2 and 11 ± 3% from C‐3. The specificity of the transfer from C‐2 increases as the internal energy of the fragmenting ions decreases, indicating that the results cannot be rationalized in terms of H/D interchanges between positions in the propyl group, but rather that the reaction involves specific, competing, H transfer reactions from each propyl position, in contrast to the high site specificity characteristic of the McLafferty rearrangement. Reaction (b) involves, almost exclusively, transfer of one hydrogen from C‐2 and one from C‐3 with only very minor participation of C‐1 hydrogens. The [C6H5COOH]+ ion produced in reaction (a) fragments further to [C6H5CO]+ + OH. and the labelling results indicate some interchange of the carboxylic hydrogen with (ortho) ring hydrogens for those ions fragmenting in the first drift region. The extent of interchange is less than that observed for fragmentation of the same ion produced by direct ionization of benzoic acid or by reaction (a) in ethyl benzoate.
The mass spectrometric [M  C2H4O]+ process in cyclic ketones. I—the nature of the C2H4O losss in bicyclo[3.3.1]nonan‐2‐one and 3‐one
Authors: Cable J. , MacLeod J. K. 
Published: May 2005

AbstractAlthough the loss of a C2H4O molecule from the molecular ions of the isomeric bicyclo[3.3.1]nonan‐2‐ and 3‐ones gives rise to the base peak at m/e 94 in both spectra, deuterium labelling results show that the processes leading to this ion are markedly different in the two cases. While the 2‐one follows essentially (>80%) the same [M  C2H4O]+ fragmentation pathway described for other 2‐alkylcyclohexanones, the 3‐one exhibits a more complex decomposition pathway in which one α‐hydrogen is transferred away from the departing C2 unit and three hydrogen atoms are subsequently rearranged to the eliminated C2H4O molecule. Similar competing fragmentation schemes have been invoked to explain deuterium labelling results in cycloheptanone, 2‐methylcycloheptanone and cyclooctanone.
Electron impact studies. CV—negative ion mass spectra of functional groups. The trimethylsilyl group. Singly charghed and doubly charged ions
Authors: Bowie John H. , Hale Barbara A. 
Published: May 2005

AbstractThe 70 eV negative ion spectra of m‐ and p‐nitrophenyltrimethylsilyl‐ethers and ‐esters show pronounced molecular anions and simple fragmentation patterns. The spectra of the corresponding ortho isomers show peaks produced by rearrangement processes; these reactions have been studied by 2H and 18O labelling. p‐Nitrobenzoyl derivatives of amino trimethylsilyl esters are of limited use for characterization of amino acids. Certain of the compounds studied produce a variety of dianions.
Zum Mechanismus Massenspektrometrischer Fragmentierungsreaktionen. XVI—hydrid‐Wanderungen bei der Fragmentierung von αω‐Dimethoxyalkyl‐Ionen
Authors: Neuert Ulrich , Grützmacher Hans‐Fr. 
Published: May 2005

AbstractDas Isomeisierungs‐und Fragmentierungsverhalten von α,w‐dimethoxyalkylionen a (CH3OCH2(CH2)n CH+OCH3, n = 1‐6) wird mit Hilfe von Deuteriummarkierungen untersucht. Es wird gezeigt, daß vor eine Methanol‐Eliminierung a durch einen Hydrid‐Übertragung von der w‐ständigen CH2‐Gruppe auf das positive Ladungszentrum am α‐C‐Atom zu a' isomerisiert und bide Methoxygruppen als Methanol abgespalten werden können. An Hand der Alken‐Eliminierung aus [a ‐CH3OH]+‐Ionen in deuterierten Derivaten von A word auf den Isomerisierungsgrad geschlossen. Bei 70 eV treten danach im Mittel bis zu drei Umlagerungsschritte auf, bei denen ejweils eine Hydrid‐Übertragung stattfindet.
The computer processing and interpretation of mass spectral information. I—The reduction of raw mass spectra to monoisotopic form
Authors: Sukharev Yu. N. , Nekrasov Yu. S. 
Published: May 2005

AbstractAn algorithm of the AELITA program (Atomic‐Element Identification) is described which enables both low and high resulution mass spectra to be processed to give the elemental composition and ion abundances of the mass spectra in monoisotopic form. The program has no limitations in respect of the elemental composition and spectrum complexity. The criterion for the choice of the most probable solution is based either on the accuracy of measurement of the peak abundances and/or the masses. The program is written in ALGOL, run on a BESM‐6 computer, and occupies about 19K core memory.
Metastable ion Characteristics. XXXXIII—separation and comparison of collisional activation spectra and metastable ion spectra
Authors: Wachs Timothy , Van De Sande C. C. , McLafferty F. W. 
Published: May 2005

AbstractAdditional ion acceleration in a separate collision chamber in the field free drift region makes it possible to add extra kinetic energy to those ions undergoing decomposition in the chamber. In this way product ions formed in the collision chamber (the [collisional activation] spectrum) can be distinguished from ions formed from decompositions in the low pressure portion of the field free drift region (the [metastable ion] spectrum). This technique is used to show that often a relatively small proportion of the ions in the [pure] collisional activation spectrum arises from processes which produce the metastable ion spectrum.
An investigation of the decomposition of the common intermediate ions produced by electron impact. IV—[C6H5CO]+ ions from several alkyl benzoates
Authors: Tajima Susumu , Azami Tadashige , Tsuchiya Toshikazu 
Published: May 2005

AbstractThe decomposition of the [C6H5CO]+ ions produced from eight alkyl benzoates by electron impact has been studied. By calculating the heat of formation of [C6H5CO]+ ions from the appearance potential value, it is shown that the ions from C6H5COOR when RH, CH3, C2H5 have some excess energy, and those where R = n‐C3H7, iso‐C3H7, n‐C4H9, iso‐C4H9, iso‐C5H11 are produced with more excess energy. It is also shown that by taking this excess energy into account, there is a linear relationship between the heat of formation of the activated complex produced in the reaction [C6H5CO]+→[C6H5]+ + CO and the vibrational degree of freedom of the neutral fragment OR.
Electron impact and chemical ionization mass spectra of alkyldiphenylphosphine oxides
Authors: Goff Susan D. , Jelus Barbara L. , Schweizer Edward E. 
Published: May 2005

AbstractIn the 70 e V electron impact mass spectra of a series of alkyldiphenylphosphine oxides (Rϕ2PO, R = Me, Et, n‐Pr, i‐Pr, n‐Bu, i‐Bu, t‐Bu, neopentyl, n‐decyl), molecular ions of low abundance are observed and [M + H]+ ions are formed to a small extent at high sample pressures. The major ions include [ϕ2PO]+, [ϕ2POH]+; [ϕ2CH2PO]+ and [ϕ2CH2POH]+ which are formed by rearrangement and cleavage processes. The chemical ionization mass spectra obtained with methane and isobutane reagents consist of [M + H]+ ions. The proton affinity of Rϕ2PO was found to be 219 ± 2.5 kcal mol−1.