The infrared laser induced isomerization of iron tetracarbonyl. The first non-Berry pseudorotation

Abstract

J. Am. Chem. SOC., 93, 4594 (1971). (31) OS~(CO)I~ A , , (e) 327 (9600): W. L. Gladfelter, unpublished observation.(32) Although the bridging carbonyls in isomer 10 are not rigorously symmetry related, the apparent coupling must arise because of their semibridging nature and the local high symmetry about Fe.Abstract: A combination of matrix isolation and IR laser induced photochemistry has been used to identify the intramolecular ligand exchange procesq occurring in the highly reactive CzC molecule Fe(C0)4, which has a structure similar to SF4. U V photolysis of Fe(C0)5, -40% enriched with 13C'80, in an Ar matrix at 20 K generated a statistical mixture of the different isomers of Fe(12C160)4-,(13C180)x. These isomers were distinguishable by their different IR absorption bands in the " C -0 stretching" region. Irradiation of the matrix with a cw C O IP. laser a t a frequency corresponding to the absorption of one particular isomer of Fe( 12C160)4-,( 13C180), promoted the selective intramolecular rearrangement of this isomer, generating a nonstatistical distribution of I3Cl8O. The permutational process occurring in this rearrangement could be identified directly from the interconversion of the three isomers of Fe(12C160)2(13C1s0)2. The laser-induced ligand exchange in Fe(C0)4 involves a non-Berry pseudorotation (permutational mode h3Ww), and differs from the thermal rearrangement of SF4, which'invalves the more familiar Berry pseudorotation (permutational mode hdWW). This difference can be rationalized using an angular overlap MO model. The photophysics of the laser rearrangement is briefly discussed. A broad asymmetric red/near-IR absorption, ; , , , 13 000 cm-', is reported for Fe(C0)d. Irradiation with light, 13 000-9000 cm-', is shown to cause nonselective isomerization of Fe('2C160)4-,(13C180)x. A possible mechanism is proposed for this near-IR isomerization.Matrix isolation is a powerful method for studying intramolecular rearrangement processes. I The particular advantage of the technique is that molecules are held rigidly,2 only rearranging when excited by either irradiatir~gl-~ or heating the matrix. In this paper we show how a combination of matrix isolation and IR laser induced photochemistry can be used to identify the rearrangement mode of the unstable CzC: molecule Fe( C 0 ) 4 .The majority of selective IR laser induced reactions which have so far been reported occur in the gas phase. They involve individual molecules overcoming a high activation barrier by rapidly absorbing many low-energy photons, usually from a high-intensity (-IOs W/cm2) COz laser beam. For example, the dissociation4 of 32SF6 requires absorption of approximately 40 photons5 at -948 cm-l. On the other hand, the energy barriers for reactions in low-temperature matrices are often relatively low and can be overcome by individual molecules absorbing only one IR photon. W e recently described6 how a low-power cw CO laser (-1 W/cm2 at -1925 cm-l) promoted the reaction between Fe(C0)4 and CH4 a t 20 K. IR laserFe(C0)4 + CH4 -+ Fe(C0)4CH4T...

Die Struktur von [Fe(CO)4] - ein neues Kapitel einer langen Geschichte

“…Ebenso bemerkenswert ist der Umstand, dass in glasartig erstarrten Kohlenwasserstoffen bei 77 K (d. h. in dem Tieftemperaturbereich, wo man Alkane vermutlich gerade noch als Lösungsmittel ansehen kann) ausschließlich 1 4 ⋅Alkan‐Addukte gebildet wurden5 und daher kein 3 4 nachgewiesen werden konnte. Spätere Matrixisolationsversuche mit kontinuierlich durchstimmbaren IR‐Lasern ergaben, dass 3 4 nach einem ungewöhnlichen Nicht‐Berry‐Pseudorotationsmechanismus isomerisieren kann, den man sich mit einer bisher einzigartigen Form des Jahn‐Teller‐Effektes erklärte 7. Dieser niederenergetische Vorgang (<2000 cm −1 ) ist in Schema dargestellt, wobei X für einen 13 C‐angereicherten Carbonylliganden steht.…”

“…Dies entspricht der bereits erwähnten Singulettroute. Die Jahn‐Teller‐Verzerrung des 1 T 2 ‐Zustandes wurde entsprechend einer für 3 4 vorgeschlagenen Nicht‐Berry‐Pseudorotation modelliert (Schema ) 7. Ideal wäre es gewesen, hätte man den Übergang des 1 A 1 ‐Zustands zum Triplett‐Grundzustand ( 1 4 → 3 4 ) als ISC‐Prozess in der Gasphase verfolgen können, aber diesmal war die Zeitauflösung (<500 ps) zu kurz; experimentell wurde die schnelle Abspaltung (3.3 ps) eines weiteren CO‐Liganden aus 1 4 unter Bildung von vermutlich angeregtem [Fe(CO) 3 ] festgestellt (der Grundzustand von C 3v ‐[Fe(CO) 3 ] ist mit großer Sicherheit ein Triplettzustand ( 3 A 2 )).…”

Overtone State‐Selective Isomerization by a Series of Picosecond Infrared Laser Pulses: Model Simulations for Be2H3D‐(C2v) → C3v)

“…Alternatively. the molecular Hamiltonian may be expressed in terms of mass-weighted coordinates (9) The corresponding potential energy surface V(" ••u 2 ) and the component ofthe dipole function along the Be-Be axis, Il(U 1 ,U 2 ) , are obtained from V(<<P I , C\l2) by means of sealings (Fig. 2).…”

Infrared spectra and molecular dynamics simulations of trans-HONO isomer in an argon matrix

“…One is the occurrence of multiple trapping sites in which molecules are isolated in di †erent microenvironments, each with their own discrete molecular spectra. The existence of multiple trapping sites in matrices has been elegantly demonstrated through site speciÐc, laser induced isomerization of by Davies et al1,2 The Fe(CO) 4 spectra of trapped molecules can yield important information on the properties of trapping sites. 3,4 There has been a growing amount of work showing that weak guestÈhost interactions a †ect the dynamics of species trapped in particular microenvironments.5,6 Such processes as photodissociation, vibrational energy transfer and relaxation, di †usion and other dynamic processes are inÑuenced by the structure of the microenvironment in which the process is taking place, hence they depend on the properties of the site.…”