346:
314:
452:, but the barriers for these processes are typically high such that these processes do not lead to line broadening. For some compounds, dynamics occur via dissociation of a ligand, giving a pentacoordinate intermediate, which is subject to the mechanisms discussed above. Yet another mechanism, exhibited by Fe(CO)
480:
96:
and typically involves recording spectra at various temperatures. In the ideal case, low temperature spectra can be assigned to the "slow exchange limit", whereas spectra recorded at higher temperatures correspond to molecules at "fast exchange limit". Typically, high temperature spectra are simpler
486:
At temperatures near 100 °C, the 500 MHz H NMR spectrum of DMF shows only one signal for the methyl groups. Near room temperature, however, separate signals are seen for the non-equivalent methyl groups. The rate of exchange can be calculated at the temperature where the two signals are
295:
579:
is the difference in Hz between the frequencies of the exchanging sites. These frequencies are obtained from the limiting low-temperature NMR spectrum. At these lower temperatures, the dynamics continue, of course, but the contribution of the dynamics to line broadening is negligible.
119:
For processes that are too slow for traditional DNMR analysis, the technique spin saturation transfer (SST, also called EXSY for exchange spectroscopy) is applicable. This magnetization transfer technique gives rate information, provided that the rates exceed
1347:
Vancea, L.; Bennett, M. J.; Jones, C. E.; Smith, R. A.; Graham, W. A. G. (1977). "Stereochemically
Nonrigid Six-Coordinate Metal Carbonyl Complexes. 1. Polytopal Rearrangement and X-Ray Structure of Tetracarbonylbis(trimethylsilyl)iron".
97:
than those recorded at low temperatures, since at high temperatures, equivalent sites are averaged out. Prior to the advent of DNMR, kinetics of reactions were measured on non-equilibrium mixtures, monitoring the approach to equilibrium.
570:
91:
Temperature dependent changes in the NMR spectra result from dynamics associated with the fluxional molecules when those dynamics proceed at rates comparable to the frequency differences observed by NMR. The experiment is called
156:
337:
and carbon-13 NMR spectra of cyclohexane show each only singlets near room temperature. At low temperatures, the singlet in the H NMR spectrum decoalesces but the C NMR spectrum remains unchanged.
644:
1414:; Davison, A.; Faller, J. W.; Lippard, S. J.; Morehouse, S. M. (1966). "Stereochemically Nonrigid Organometallic Compounds. I. π-Cyclopentadienyliron Dicarbonyl σ-Cyclopentadiene".
365:, even at temperatures as low as −100 °C, fails to distinguish the axial from the equatorial fluorine environments. The apparent equivalency arises from the low barrier for
1072:
Casey H. Londergan; Clifford P. Kubiak (2003). "Electron
Transfer and Dynamic Infrared-Band Coalescence: It Looks Like Dynamic NMR Spectroscopy, but a Billion Times Faster".
362:
358:
329:. Carbon–hydrogen bonds that are axial in one configuration become equatorial in the other, and vice versa. At room temperature the two chair conformations rapidly
1132:
100:
Many molecular processes exhibit fluxionality that can be probed on the NMR time scale. Beyond the examples highlighted below, other classic examples include the
493:
385:: only one signal is observed in the C NMR spectrum near room temperature) whereas at low temperatures, two signals in a 2:3 ratio can be resolved. In
1037:
Jarek, R. L.; Flesher, R. J.; Shin, S. K. (1997). "Kinetics of
Internal Rotation of N,N-Dimethylacetamide: A Spin-Saturation Transfer Experiment".
1331:
345:
290:{\displaystyle k\sim \Delta \nu _{\circ }\sim 2(10\mathrm {cm} ^{-1})(300\cdot 10^{8}\mathrm {cm/s} )\sim 6\times 10^{11}\mathrm {s} ^{-1}\cdot }
300:
Clearly, processes that induce line-broadening on the IR time-scale must be much more rapid than the cases that exchange on the NMR time scale.
890:
464:
and related hydride complexes, is intramolecular scrambling of ligands over the faces of the tetrahedron defined by the four CO ligands.
55:, the term fluxional depends on the context and the method used to assess the dynamics. Often, a molecule is considered fluxional if its
444:, featuring close-packed array of six ligating atoms surrounding a central atom. Such compounds do rearrange intramolecularly via the
1449:
1375:
H. S. Gutowsky; C. H. Holm (1956). "Rate
Processes and Nuclear Magnetic Resonance Spectra. II. Hindered Internal Rotation of Amides".
361:
consists of a P-coupled doublet, indicating that the equatorial and axial fluorine centers interchange rapidly on the NMR timescale.
1467:
1116:
951:
864:
867:
683:
64:
416:
there is no rigid molecular structure; the H atoms are always in motion. More precisely, the spatial distribution of protons in
852:: The use of permutation-inversion groups for the symmetry classification of the states of fluxional (or non-rigid) molecules.
1154:-butylcyclohexane by Dynamic NMR Spectroscopy and Computational Methods. Observation of Chair and Twist-Boat Conformations".
67:) due to chemical exchange. In some cases, where the rates are slow, fluxionality is not detected spectroscopically, but by
441:
593:
47:
interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in some respects, e.g.
150:. Application of the equation for coalescence of two signals separated by 10 cm gives the following result:
354:
109:
48:
733:
ligand splits at low temperatures owing to the slow hopping of the Fe center from carbon to carbon in the η-C
440:
While nonrigidity is common for pentacoordinate species, six-coordinate species typically adopt a more rigid
1190:
Gutowsky, H. S.; McCall, D. W.; Slichter, C. P. (1953). "Nuclear
Magnetic Resonance Multiplets in Liquids".
80:
139:
1328:
487:
just merged. This "coalescence temperature" depends on the measuring field. The relevant equation is:
1384:
1245:
1200:
1046:
1011:
833:
386:
330:
750:
791:
1271:
1126:
766:
374:
101:
1445:
1306:
1263:
1236:
1172:
1112:
1089:
947:
886:
860:
781:
741:
ligand. Two mechanisms have been proposed, with the consensus favoring the 1,2 shift pathway.
473:
143:
68:
24:
875:, Wiley, New York, 1975 (reprinted by Dover 1992), describing the term "semi-rigid molecule".
1423:
1411:
1392:
1357:
1298:
1253:
1208:
1164:
1081:
1054:
1019:
986:
922:
841:
121:
52:
880:
1335:
786:
370:
1289:
Thompson, KC; Crittenden, DL; Jordan, MJ (2005). "CH5+: Chemistry's chameleon unmasked".
1388:
1249:
1204:
1050:
1015:
837:
1192:
771:
565:{\displaystyle k={\frac {\pi \Delta \nu _{\circ }}{2^{1/2}}}\sim 2\Delta \nu _{\circ }}
445:
366:
313:
1461:
1156:
709:
At 30 °C, the H NMR spectrum shows only two peaks, one typical (δ5.6) of the η-C
413:
147:
60:
56:
1275:
1226:
Asvany, O.; Kumar P, P.; Redlich, B.; Hegemann, I.; Schlemmer, S.; Marx, D. (2005).
990:
776:
449:
393:), a similar pattern is observed even though this compound has only four ligands.
322:
113:
1440:
Robert B. Jordan, Reaction
Mechanisms of Inorganic and Organometallic Systems (
914:
373:, by which the axial and equatorial fluorine atoms rapidly exchange positions.
846:
821:
755:
334:
105:
138:
Although less common, some dynamics are also observable on the time-scale of
1258:
1227:
927:
761:
647:
397:
326:
20:
1310:
1267:
1176:
1093:
1085:
40:
1427:
1361:
981:
John W. Faller "Stereochemical
Nonrigidity of Organometallic Complexes"
1396:
1302:
1212:
1168:
1058:
1023:
767:
Molecular symmetry § Molecular rotation and molecular nonrigidity
317:
Cyclohexane chair flip (ring inversion) reaction via boat conformation
1329:
http://www.theochem.ruhr-uni-bochum.de/research/marx/topic4b.en.html
1146:
Gill, G.; Pawar, D. M.; Noe, E. A. (2005). "Conformational Study of
682:
479:
681:
312:
44:
16:
Molecules whose atoms interchange between symmetric positions
478:
344:
83:
exhibit fluxionality. Fluxionality is, however, pervasive.
816:, McGraw-Hill, New York, 1955 (reprinted by Dover 1980)
321:
The interconversion of equivalent chair conformers of
596:
496:
159:
43:
that undergo dynamics such that some or all of their
983:
Encyclopedia of
Inorganic and Bioinorganic Chemistry
639:{\displaystyle k\sim 2(500)=1000\mathrm {s} ^{-1}}
638:
564:
289:
428:is many times broader than its parent molecule CH
341:Berry pseudorotation of pentacoordinate compounds
1002:Bryant, Robert G. (1983). "The NMR time scale".
1228:"Understanding the Infrared Spectrum of Bare CH
946:(2nd ed.). Philadelphia: W. B. Saunders.
812:E. B. Wilson, J. C. Decius, and P. C. Cross,
472:A classic example of a fluxional molecule is
8:
822:"The symmetry groups of non-rigid molecules"
679:exhibits the phenomenon of "ring whizzing".
325:(and many other cyclic compounds) is called
762:Hapticity § Hapticity and fluxionality
1131:: CS1 maint: location missing publisher (
885:(2 ed.). Ottawa: NRC Research Press.
1257:
926:
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275:
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231:
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192:
173:
158:
807:Molecular Vibrational-Rotational Spectra
686:The structure of the ring whizzer Fe(η-C
906:
1323:For an animation of the dynamics of CH
1124:
1111:(2nd ed.). Oxford. p. 373.
879:Philip R. Bunker; Per Jensen (2006).
353:A prototypical fluxional molecule is
7:
396:A well-studied fluxional ion is the
882:Molecular Symmetry and Spectroscopy
857:Fundamentals of Molecular Symmetry
623:
549:
509:
349:Iron-pentacarbonyl-Berry-mechanism
271:
243:
235:
232:
196:
193:
166:
14:
725:. The singlet assigned to the η-C
381:) follows the pattern set for PF
65:Heisenberg uncertainty principle
991:10.1002/9781119951438.eibc0211
820:Longuet-Higgins, H.C. (1963).
612:
606:
247:
212:
209:
185:
1:
1442:Topics in Inorganic Chemistry
1074:Chemistry: A European Journal
1039:Journal of Chemical Education
1004:Journal of Chemical Education
944:Physical Methods in Chemistry
805:D. Papoušek and M. R. Aliev,
442:octahedral molecular geometry
309:Cyclohexane and related rings
63:(beyond that dictated by the
855:P. R. Bunker and P. Jensen,
363:Fluorine-19 NMR spectroscopy
146:in a mixed-valence dimer of
985:2011, Wiley-VCH, Weinheim.
1484:
942:Drago, Russell S. (1977).
873:Molecular Rotation Spectra
717:and the other assigned η-C
847:10.1080/00268976300100501
809:Elsevier, Amsterdam, 1982
1468:Chemical bond properties
969:Dynamic NMR Spectroscopy
355:phosphorus pentafluoride
81:organometallic compounds
1259:10.1126/science.1113729
928:10.1351/goldbook.F02463
1086:10.1002/chem.200305028
758:, a fluxional molecule
706:
640:
587:= 1ppm @ 500 MHz
566:
483:
436:Six-coordinate species
350:
318:
291:
967:J. Sandström (1982).
685:
646:(ca. 0.5 millisecond
641:
567:
482:
348:
316:
292:
75:Spectroscopic studies
1410:Bennett, Jr. M. J.;
814:Molecular Vibrations
594:
494:
387:sulfur tetrafluoride
157:
1428:10.1021/ja00971a012
1389:1956JChPh..25.1228G
1362:10.1021/ic50170a035
1350:Inorganic Chemistry
1250:2005Sci...309.1219A
1244:(5738): 1219–1222.
1205:1953JChPh..21..279G
1163:(26): 10726–10731.
1107:J, Clayden (2003).
1051:1997JChEd..74..978J
1016:1983JChEd..60..933B
838:1963MolPh...6..445L
751:Pyramidal inversion
659:The compound Fe(η-C
71:and other methods.
59:signature exhibits
1334:2007-12-24 at the
859:, CRC Press, 1998
707:
636:
583:For example, if Δν
562:
484:
375:Iron pentacarbonyl
351:
319:
287:
102:Cope rearrangement
1397:10.1063/1.1743184
1303:10.1021/ja0482280
1297:(13): 4954–4958.
1213:10.1063/1.1698874
1169:10.1021/jo051654z
1109:Organic chemistry
1059:10.1021/ed074p978
1024:10.1021/ed060p933
971:. Academic Press.
892:978-0-660-19628-2
826:Molecular Physics
782:Bartell mechanism
541:
474:dimethylformamide
468:Dimethylformamide
144:electron transfer
142:. One example is
69:isotopic labeling
53:organic compounds
25:molecular physics
1475:
1452:
1438:
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1416:J. Am. Chem. Soc
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110:chair inversion
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36:
32:
28:
18:
1010:(11): 933.
915:"Fluxional"
432:, methane.
331:equilibrate
323:cyclohexane
114:cyclohexane
1199:(2): 279.
1045:(8): 978.
901:References
756:Bullvalene
412:. Even at
106:bullvalene
1444:), 2007.
1127:cite book
648:half-life
629:−
601:∼
558:∘
554:ν
550:Δ
544:∼
518:∘
514:ν
510:Δ
507:π
398:methanium
285:⋅
277:−
257:×
251:∼
219:⋅
202:−
180:∼
175:∘
171:ν
167:Δ
164:∼
41:molecules
37:molecules
33:non-rigid
29:fluxional
21:chemistry
1462:Category
1332:Archived
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1276:28745636
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1177:16355992
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1012:Bibcode
834:Bibcode
476:(DMF).
377:(Fe(CO)
335:proton-
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45:atoms
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1420:1966
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