Comment on “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix”

Alabugin, Igor V.; Gold, Brian; Shatruk, Michael; Kovnir, Kirill

doi:10.1126/science.1196188

Cited by 32 publications

(25 citation statements)

References 17 publications

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“…All these factors lead to what is called "poor" data quality. Data quality can indeed lead to alternative interpretations, as illustrated by the comments of Scheschkewitz (2) and Alabugin et al (3). However, we show below that our interpretation is more likely.…”

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confidence: 48%

“…Scheschkewitz (2) and Alabugin et al (3) contend that our structures instead correspond with a bicyclic b-lactone intermediate. Our view on the importance of the quality of the x-ray data on structure resolution and the view of the Technical Comment authors coincide.…”

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confidence: 99%

“…For a more lengthy discussion about the quality of the x-ray data questioned by Alabugin et al (3) and more comments about table S1, see the Supporting Online Material of this Response.…”

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confidence: 99%

“…2. Moreover, one might argue that the CO 2 molecule could not be in such vicinity with Me 2 CBD R and therefore is not present in the elementary cell where Me 2 CBD R is present, because of some fairly short distances as outlined in (2,3). Indeed, alternative refinements show that if the occupancy of the O7-C2-O1 part is refined freely, it drops down from 1.0 to 0.83, as compared with an occupation of 0.63 for Me 2 ).…”

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confidence: 99%

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Response to Comments on “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix”

Legrand

Lee

Barboiu

2010

Science

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Scheschkewitz and Alabugin et al. suggest that photolysis under confinement in a crystalline matrix of 4,6-dimethyl-a-pyrone does not yield the crystal structure of 1,3-dimethylcyclobutadiene (Me 2 CBD) as we reported, but rather that of a 4,6-dimethyl-b-lactone intermediate. We provide arguments that the square-planar Me 2 CBD S /CO 2 complex and the rectangular-bent Me 2 CBD R molecule are stabilized under confinement by the guanidinium-sulfonate-calixarene host matrix used in our study. W e recently reported the single-crystal x-ray structure of 1,3-dimethylcyclobutadiene synthesized within a guanidinium-sulfonate-calixarene (G 4 C) crystalline matrix (1). Scheschkewitz (2) and Alabugin et al.(3) contend that our structures instead correspond with a bicyclic b-lactone intermediate. Our view on the importance of the quality of the x-ray data on structure resolution and the view of the Technical Comment authors coincide. In their comments, Scheschkewitz and Alabugin et al. consider our work as an unconvincing attempt, mainly due to the presumed poor quality of the experimental x-ray data. They fail, however, to place the crystallographic work in the particular context of highly advanced and challenging chemistry of unstable or reactive host-guest crystalline systems. A number of very comparable studies, albeit on other systems, have preceded our work (4-7), and the problems encountered are virtually the same as ours: resolved and unresolved disorder leading to results far from optimal data quality. Data and model quality are not expressed in a satisfactory way by just one or two numbers or parameters. Instead, a more or less complete set of parameters needs to be given to characterize the quality of data and model. Table S1 summarizes the different quality parameters for these very comparable studies (1, 4-7) in the field of reactive host-guest crystalline systems. Our data are no worse than these studies, if not better, and in all cases the conclusion would be that the data warrants the structural model described in the paper. Our opinion is that all previous important contributions related to unstable molecules sequestered in porous crystals can reach their full potential.The difficulties of experiments involving hostguest crystalline systems are related to the following factors: (i) in the case of nonreactive molecules (4, 5), the multiple orientations of the guest in a host cavity induce disorder; (ii) in the case of reactional processes, inconsistent diffusion of the reagents or unequal distribution of the irradiation energy over the whole volume of the crystal also leads to disorder (6, 7); and (iii) the preservation of the crystallinity of the hostmatrix may be affected during the reactional process by a loss of water or solvent molecules. All these factors lead to what is called "poor" data quality. Data quality can indeed lead to alternative interpretations, as illustrated by the comments of Scheschkewitz (2) and Alabugin et al. (3). However, we show below that our interpretation is more likely.Schesch...

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confidence: 48%

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confidence: 99%

“…For a more lengthy discussion about the quality of the x-ray data questioned by Alabugin et al (3) and more comments about table S1, see the Supporting Online Material of this Response.…”

mentioning

confidence: 99%

mentioning

confidence: 99%

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Response to Comments on “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix”

Legrand

Lee

Barboiu

2010

Science

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“…[12,13] However, this finding initiated heated debate in literature on whether the measured crystal structure corresponds to 1,3-dimethylcyclobutadiene at al. [14][15][16][17] Valence bond isomerism of cyclobutadiene has also aroused considerable interest of theoretical chemists, to mention only the most recent high level computational studies by Balkova and Bartlett, [12] Sancho-Garcia et al, [18] Shen et al, [19] Lefrancois et al, [20] and our research group. [13] We have shown that the barrier for automerization reaction of the parent CBD molecule obtained at the MR-AQCC level of theory including extrapolations to the basis set limit is 6.3 kcal mol −1 .…”

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confidence: 99%

Multireference Configuration Interaction Methods – An Application to the Valence Isomerism in Cyclobutadieno-p-benzoquinone and its Diprotonated Form

Vazdar

Eckert‐Maksić

2015

Croat. Chem. Acta

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Multireference averaged quadratic coupled cluster (MR-AQCC) calculations for cyclobutadieno-p-benzoquinone indicate that valence bond isomers 1a and 1b can exist as distinct species. The energy barrier height for their interconversion are 4.6 and 4.5 kcal mol −1, respectively, what is by ca. 2 kcal mol −1 lower than in the parent cyclobutadiene, implying that they could perhaps exist only under extreme conditions, namely at very low temperatures. For double protonated cyclobutadieno-p-benzoquinone, the CASSCF calculations erroneously predict existence of two valence isomers, 2a and 2b, whereas the MR-AQCC calculations reveal that geometry of the double protonated species could be best described by structure 2b. This nicely illustrates the crucial role of dynamic correlation and the need for using a highly-correlated theoretical method including geometry optimization in studied molecules.

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Neue Technologie im Nanometer‐Maßstab

Groß¹

2012

Von Geckos, Garn Und Goldwasser

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Comment on “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix”

Cited by 32 publications

References 17 publications

Response to Comments on “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix”

Response to Comments on “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix”

Multireference Configuration Interaction Methods – An Application to the Valence Isomerism in Cyclobutadieno-p-benzoquinone and its Diprotonated Form

Neue Technologie im Nanometer‐Maßstab

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