Tải bản đầy đủ - 0 (trang)
G. Spectral Evolution in Associated Liquids

G. Spectral Evolution in Associated Liquids

Tải bản đầy đủ - 0trang

584



Iwaki et al.



environments. There is a direct inverse correlation between the strength of

the hydrogen bond and the O–H stretching frequency: the lowest frequency

O–H stretch is associated with the strongest hydrogen bond and vice

versa (86,87). Pumping the O–H stretch of associated alcohol oligomers

is known to result in a vibrational predissociation process which breaks

the hydrogen bond (88–91), so the transition from D 1 ! 0 is accompanied by hydrogen bond cleavage. Vibrational predissociation in associated

alcohols reduces the vibrational lifetime from tens or more picoseconds to

¾1 ps (88–92).

Figure 22 shows some data on methanol pumped in the O–H

stretching region by a 35 cm 1 wide pulse at 3400 cm 1 . When the pump

and probe pulses are time coincident, a coherent artifact is observed at an

anti-Stokes shift of 3400 cm 1 . By about 2 ps this artifact has vanished



Figure 22 O–H stretching region of neat methanol with 3400 cm 1 pumping.

The peak at 3400 cm 1 is a coherent artifact. After the artifact has disappeared, the

spectrum of O–H stretching excitations decays and the peak of the OH stretching

spectrum shifts to higher energy. Methanol molecules with higher frequency O–H

stretching transitions and weaker hydrogen bonding decay more slowly. (From

L. K. Iwaki et al., unpublished.)



Copyright © 2001 by Taylor & Francis Group, LLC



Ultrafast IR-Raman Spectroscopy



585



and the spectrum of the remaining methanol excitations can easily be seen.

The O–H stretching population decays in just a few picoseconds. During

this short lifetime, the O–H spectrum undergoes a time-dependent change

in shape, with the spectral peak continuously moving toward the region of

higher energy. A similar motion toward higher O–H stretching frequency

is seen in Fig. 11. There is no noticeable spectral evolution in the daughter

C–H stretch excitations, between 2750 and 3000 cm 1 , which are excited

by O–H stretch decay. The shape change in the O–H spectrum is attributed

to frequency-dependent vibrational relaxation rates. Decay of vibrational

excitations at the higher frequency end of the spectrum appears to be slower

than at the lower frequency end. Therefore, methanol molecules with the

weakest hydrogen bonds have the longest VER lifetimes.

This effect is understood by considering the dispersal of the

¾3400 cm 1 of energy liberated in the D 1 ! 0 transition. Figure 22

shows that very little of the O–H stretching excitation is transferred

to the nearby C–H stretching region. Instead, the energy is used to

populate fundamental excitations of vibrations in the 1000–1500 cm 1

range, such as O–H bending, C–H bending, and C–O stretching

excitations (L. K. Iwaki et al., unpublished). Exciting these vibrations

requires the simultaneous emission of a large number of phonons equal

to 1000–2000 cm 1 . This would be a high-order inefficient multiphonon

process. Populating these vibrations while simultaneously breaking a

hydrogen bond is a much faster and more efficient process, because

breaking a hydrogen bond in methanol releases 1000–2000 cm 1 of

energy (87).

V. SUMMARY AND CONCLUSIONS



The IR-Raman technique has been used for over 20 years, but only in the

last few years has suitable laser instrumentation been developed to monitor

the flow of vibrational energy throughout polyatomic molecules in liquids.

The new results are remarkable. For example, although it had been known

since the earliest transient spontaneous Raman experiments that C–H and

O–H stretch decay takes a few picoseconds, now we can see that the VC

process initiated by pumping higher frequency vibrations can last for more

than 150 ps (e.g., ACN in Fig. 16). New results also show us the need to

move beyond the vibrational cascade picture of VC. A quite new development is the ability to study the interactions between vibrations on the

same molecule via combination band pumping. Although experiments so

far have been mainly limited to polyatomic liquids consisting of relatively



Copyright © 2001 by Taylor & Francis Group, LLC



586



Iwaki et al.



small molecules, the CCl4 results suggest a new way of studying VC

in larger molecules — or even macromolecules, molecular aggregates, or

nanoparticles — by monitoring the flow of energy from the large molecules

to CCl4 or another molecular probe in the surroundings.

The IR-Raman technique is one of several new ultrafast twodimensional vibrational spectroscopies (78,93–95), which have begun to

revolutionize our understanding of condensed phase vibrational dynamics.

Although IR-Raman methods are complementary to IR pump-probe

measurements, especially two-color pump-probe, a stellar advantage of the

IR-Raman technique is the ability to simultaneously obtaining the entire

vibrational spectrum with an optical array, as illustrated in Figs. 9, 11,

20, and 22. The big disadvantage of the IR-Raman technique is the weak

Raman signal, which is easily overwhelmed by even tiny amounts of optical

background. However, the IR-Raman technique allows us to study the time

evolution of vibrational populations even at lower frequencies inaccessible

to today’s ultrashort IR lasers and to study the time evolution of the optical

lineshape with unprecedented accuracy.

ACKNOWLEDGMENTS



This work was supported by Air Force Office of Scientific Research contract

F49620-97-1-0056, U.S. Army Research Office contract DAAH04-96-10038, and National Science Foundation grant DMR-9714843. L.K.I. and

S.T.R. acknowledge support from an AASERT fellowship, DAAG55-98-10191, from the Army Research Office.

REFERENCES

1.



2.

3.

4.

5.

6.



Spanner K, Laubereau A, Kaiser W. Vibrational energy redistribution of polyatomic molecules in liquids after ultrashort infrared excitation. Chem Phys

Lett 1976; 44:88–92.

Laubereau A, Kaiser W. Vibrational dynamics of liquids and solids investigated by picosecond light pulses. Rev Mod Phys 1978; 50:607–665.

Dlott DD. Dynamics of molecular crystal vibrations. In: Yen W, ed. Laser

Spectroscopy of Solids II. Berlin: Springer-Verlag, 1988:167–200.

Hill JR, Dlott DD. A model for ultrafast vibrational cooling in molecular

crystals. J Chem Phys 1988; 89:830–841.

Hill JR, Dlott DD. Theory of vibrational cooling in molecular crystals: application to crystalline naphthalene. J Chem Phys 1988; 89:842–858.

Flynn GW, Parmenter CS, Wodtke AM. Vibrational energy transfer. J Phys

Chem 1996; 100:12817–12838.



Copyright © 2001 by Taylor & Francis Group, LLC



Ultrafast IR-Raman Spectroscopy



587



7. Chesnoy J, Gale GM. Vibrational energy relaxation in liquids. Ann Phys Fr

1984; 9:893949.

8. Schroeder J, Troe J, Văohringer P. Photoisomerization of trans-stilbene in

compressed solvents: Kramers-turnover and solvent induced barrier shift. Z

Phys Chem 1995; 188:287–306.

9. Schwarzer D, Troe J, Votsmeier M, Zerezke M. Collisional deactivation of

vibrationally highly excited azulene in compressed liquids and supercritical

fluids. J Chem Phys 1996; 105:3121–3131.

10. Schwarzer D, Troe J, Zerezke M. The role of local density in the collisional

deactivation of vibrationally highly excited azulene in supercritical fluids. J

Chem Phys 1997; 107:8380–8390.

11. Cherayil BJ, Fayer MD. Vibrational relaxation in supercritical fluids near the

critical point. J Chem Phys 1997; 107:7642–7650.

12. Myers DJ, Urdahl RS, Cherayil BJ, Fayer MD. Temperature dependence of

vibrational lifetimes at the critical density in supercritical mixtures. J Phys

Chem 1997; 107:9741–9748.

13. Myers DJ, Chen S, Shigeiwa M, Cherayil BJ, Fayer MD. Temperature dependent vibrational lifetimes in supercritical fluids near the critical point. J Chem

Phys 1998; 109:5971–5979.

14. Harris CB, Smith DE, Russell DJ. Vibrational relaxation of diatomic

molecules in liquids. Chem Rev 1990; 90:481–488.

15. Paige ME, Harris CB. A generic test of gas phase isolated binary collision

theories for vibrational relaxation at liquid state densities based on the rescaling properties of collision frequencies. J Chem Phys 1990; 93:3712–3713.

16. Paige ME, Harris CB. Ultrafast studies of chemical reactions in liquids:

validity of gas phase vibrational relaxation models and density dependence of

bound electronic state lifetimes. Chem Phys 1990; 149:37–62.

17. Russell DJ, Harris CB. Vibrational relaxation in simple fluids: a comparison

of experimental results to the predictions of isolated binary collision theory.

Chem Phys 1994; 183:325–333.

18. Stratt RM, Maroncelli M. Nonreactive dynamics in solution: the emerging

molecular view of solvation dynamics and vibrational relaxation. J Phys Chem

1996; 100:12981–12996.

19. Voth GA, Hochstrasser RM. Transition state dynamics and relaxation

processes in solutions: a frontier of physical chemistry. J Phys Chem 1996;

100:13034–13049.

20. Brueck SRJ, Osgood Jr. RM. Vibrational energy relaxation in liquid N2 CO

mixtures. Chem Phys Lett 1976; 39:568–572.

21. Hoffman GJ, Imre DG, Zadoyan R, Schwentner N, Apkarian VA. Relaxation

dynamics in the B(1/2) and C(3/2) charge transfer states of XeF in solid Ar.

J Chem Phys 1993; 98:9233–9240.

22. Oxtoby DW. Vibrational population relaxation in liquids. In: Jortner J,

Levine RD, Rice SA, eds. Photoselective Chemistry Part 2. Vol. 47. New

York: Wiley, 1981:487–519.



Copyright © 2001 by Taylor & Francis Group, LLC



588



Iwaki et al.



23. Vergeles M, Szamel G. A theory for dynamic friction on a molecular bond. J

Chem Phys 1999; 110:6827–6835.

24. Grote RF, Hynes JT. The stable states picture of chemical reactions. II. Rate

constants for condensed and gas phase reaction models. J Chem Phys 1980;

73:2715–2732.

25. Grote RF, Hynes JT. Reactive modes in condensed phase reactions. J Chem

Phys 1981; 74:4465–4475.

26. Kramers HA. Brownian motion in a field of force and the diffusion model of

chemical reactions. Physica 1940; VII:284–304.

27. Lee M, Holtom GR, Hochstrasser RM. Observation of the Kramers turnover

region in the isomerism of trans-stilbene in uid ethane. Chem Phys Lett

1985; 118:359363.

28. Fleming G, Hăanggi P. Activated Barrier Crossing. River Edge, NJ: World

Scientific, 1993.

29. Nikowa L, Schwarzer D, Troe J. Transient hot UV spectra in the collisional

deactivation of highly excited trans-stilbene in liquid solvents. Chem Phys

Lett 1995; 233:303–308.

30. Hasha DL, Eguchi T, Jonas J. Dynamical effects on conformational isomerization of cyclohexane. J Chem Phys 1981; 75:1571–1573.

31. Hasha DL, Eguchi T, Jonas J. High-pressure NMR study of dynamical effects

on conformational isomerization of cyclohexane. J Am Chem Soc 1982;

104:2290–2296.

32. Laubereau A, von der Linde D, Kaiser W. Direct measurement of the vibrational lifetimes of molecules in liquids. Phys Rev Lett 1972; 28:1162–1165.

33. Alfano RR, Shapiro SL. Establishment of a molecular-vibration decay route

in a liquid. Phys Rev Lett 1972; 29:1655–1658.

34. Seilmeier A, Kaiser W. Ultrashort intramolecular and intermolecular vibrational energy transfer of polyatomic molecules in liquids. In: Kaiser W, ed.

Ultrashort Laser Pulses and Applications. Vol. 60. Berlin: Springer-Verlag,

1988: 279–315.

35. Oxtoby DW. Vibrational relaxation in liquids. Ann Rev Phys Chem 1981;

32:77–101.

36. Laubereau A, Greiter L, Kaiser W. Intense tunable picosecond pulses in the

infrared. Appl Phys Lett 1974; 25:87–89.

37. Zinth W, Kolmeder C, Benna B, Irgens-Defregger A, Fischer SF, Kaiser W.

Fast and exceptionally slow vibrational energy transfer in acetylene and phenylacetylene in solution. J Chem Phys 1983; 78:3916–3921.

38. Ambroseo JR, Hochstrasser RM. Pathways of relaxation of the N-H stretching

vibration of pyrrole in liquids. J Chem Phys 1988; 89:5956–5957.

39. Gottfried NH, Kaiser W. Redistribution of vibrational energy in naphthalene

and anthracene studied in liquid solution. Chem Phys Lett 1983; 101:331–336.

40. Tokmakoff A, Sauter B, Kwok AS, Fayer MD. Phonon-induced scattering

between vibrations and multiphoton vibrational up-pumping in liquid solution.

Chem Phys Lett 1994; 221:412–418.



Copyright © 2001 by Taylor & Francis Group, LLC



Ultrafast IR-Raman Spectroscopy



589



41. Chen S, Hong X, Hill JR, Dlott DD. Ultrafast energy transfer in high explosives: vibrational cooling. J Phys Chem 1995; 99:4525–4530.

42. Hong X, Chen S, Dlott DD. Ultrafast mode-specific intermolecular vibrational

energy transfer to liquid nitromethane. J Phys Chem 1995; 99:9102–9109.

43. Hofmann M, Zăurl R, Graener H. Polarization effects in time resolved incoherent anti-Stokes Raman spectroscopy. J Chem Phys 1996; 105:6141–6146.

44. Graener H, Zăurl R, Hofmann M. Vibrational relaxation of liquid chloroform.

J Phys Chem 1997; 101:1745–1749.

45. De`ak JC, Iwaki LK, Dlott DD. High power picosecond mid-infrared optical

parametric amplifier for infrared-Raman spectroscopy. Opt Lett 1997;

22:1796–1798.

46. De`ak JC, Iwaki LK, Dlott DD. Vibrational energy relaxation of polyatomic

molecules in liquids: acetonitrile. J Phys Chem 1998; 102:8193–8201.

47. De`ak JC, Iwaki LK, Dlott DD. When vibrations interact: ultrafast energy

relaxation of vibrational pairs in polyatomic liquids. Chem Phys Lett 1998;

293:405–411.

48. De`ak JC, Iwaki LK, Dlott DD. Vibrational energy redistribution in polyatomic

liquids: ultrafast IR-Raman spectroscopy of nitromethane. J Phys Chem A

103:971–979.

49. Iwaki LK, De`ak JC, Rhea ST, Dlott DD. Vibrational energy redistribution in

liquid benzene. Chem Phys Lett 1999; 303:176–182.

50. Iwaki L, Dlott DD. Vibrational energy transfer in condensed phases. In:

Moore JH, Spencer ND, eds. Encyclopedia of Chemical Physics and Physical

Chemistry. Philadelphia: Institute of Physics, 2000.

51. Berg M, Vanden Bout DA. Ultrafast Raman echo measurements of vibrational

dephasing and the nature of solvent-solute interactions. Acc Chem Res 1997;

30:65–71.

52. Everitt KF, Egorov SA, Skinner JL. Vibrational energy relaxation in liquid

oxygen. Chem Phys 1998; 235:115–122.

53. Velsko S, Oxtoby DW. Vibrational energy relaxation in liquids. J Chem Phys

1980; 72:2260–2263.

54. Moore P, Tokmakoff A, Keyes T, Fayer MD. The low frequency density of

states and vibrational population dynamics of polyatomic molecules in liquids.

J Chem Phys 1995; 103:3325–3334.

55. Seeley G, Keyes T. Normal-mode analysis of liquid-state dynamics. J Chem

Phys 1989; 91:5581–5586.

56. Xu B-C, Stratt RM. Liquid theory for band structure in a liquid. II. p Orbitals

and phonons. J Chem Phys 1990; 92:1923–1935.

57. Kenkre VM, Tokmakoff A, Fayer MD. Theory of vibrational relaxation of

polyatomic molecules in liquids. J Chem Phys 1994; 101:10618–10629.

58. Goodyear G, Stratt RM. The short-time intramolecular dynamics of solutes

in liquids. I. An instantaneous-normal-mode theory for friction. J Chem Phys

1996; 105:10050–10071.



Copyright © 2001 by Taylor & Francis Group, LLC



590



Iwaki et al.



59. Goodyear G, Larsen RE, Stratt RM. Molecular origin of friction in liquids.

Phys Rev Lett 1996; 76:243–246.

60. Bader JS, Berne BJ. Quantum and classical rates for classical simulations. J

Chem Phys 1994; 100:8359–8366.

61. Egelstaff PA. Neutron scattering studies of liquid diffusion. Adv Phys 1962;

11:203–232.

62. Califano S, Schettino V, Neto N. Lattice Dynamics of Molecular Crystals.

Berlin: Springer-Verlag, 1981.

63. Dlott DD, Fayer MD. Shocked molecular solids: vibrational up pumping,

defect hot spot formation, and the onset of chemistry. J Chem Phys 1990;

92:3798–3812.

64. Nitzan A, Jortner J. Vibrational relaxation of a molecule in a dense medium.

Molec Phys 1973; 25:713–734.

´

65. Bokhenkov EL,

Rodina EM, Sheka EF, Natkaniec I. Inelastic incoherent

neutron scattering spectra at different temperatures and computer experiment

for external phonon modes of naphthalene crystals. Phys Status Solidi B 1978;

85:331–342.

´

66. Belushkin AV, Bokhenkov EL,

Kolesnkiov AI, Natkaniec I, Righini R,

Sheka EF. Spectrum of external phonons of a naphthalene crystal at 5K. Sov.

Phys Solid State 1991; 23:1529–1533.

67. Hill JR, Chronister EL, Chang T-C, Kim H, Postlewaite JC, Dlott DD. Vibrational relaxation and vibrational cooling in low temperature molecular crystals.

J Chem Phys 1988; 88:949–967.

68. Backus S, Durfee III CG, Murnane MM, Kapteyn HC. High power ultrafast

lasers. Rev Sci Instrum 1998; 69:1207–1223.

69. Rudd JV, Korn G, Kane S, Squier J, Mourou G, Bado P. Chirped-pulse

amplification of 55 fs pulses at a 1 kHz repetition rate in a Ti:A12 O3

regenerative amplifier. Opt Lett 1993; 18:2044–2046.

70. Raoult F, Boscheron ACL, Husson D, Sauteret C, Modena A, Malka V,

Dorchies F, Migus A. Efficient generation of narrow-bandwidth picosecond

pulses by frequency doubling of femtosecond chirped pulses. 1998; Opt Lett

23:1117–1119.

71. Gabl EF, Walker DR, Pang Y. Regenerative amplifier incorporating a spectral

filter within the resonant cavity. USA Patent 5,572,358 (1996).

72. Zhang J-Y, Huang JY, Shen YR. Optical Parametric Generation and Amplification. Harwood Academic Publishers, 1995.

73. Dierlein JD, Vanherzeele H, Ballman AA. Linear and nonlinear optical properties of flux-grown KTiOAsO4 . Appl Phys Lett 1989; 54:783–785.

74. Petrov V, Noack F. Mid-infrared femtosecond optical parametric amplification in potassium niobate. Opt Lett 1996; 19:1576–1578.

75. Gragson DE, Alavi DS, Richmond GL. Tunable picosecond infrared laser

system based on parametric amplification in KTP with a Ti:sapphire amplifier.

Opt Lett 1995; 20:1991–1993.



Copyright © 2001 by Taylor & Francis Group, LLC



Ultrafast IR-Raman Spectroscopy



591



76. Schrader B. Raman/Infrared Atlas of Organic Compounds. Weinheim: VCH,

1989.

77. Alfano RR. The Supercontinuum Laser Source. New York: Springer-Verlag,

1989.

78. Mukamel S. Principles of Nonlinear Optical Spectroscopy. New York: Oxford

University Press, 1995.

79. Shen YR. Surfaces probed by nonlinear optics. Surface Sci 1994;

299/300:551–562.

80. Shen YR. Surface properties probed by second-harmonic and sum-frequency

generation. Nature 1989; 337:519–525.

81. Fendt A, Fischer SF, Kaiser W. Vibrational lifetime and Fermi resonance in

polyatomic molecules. Chem Phys 1981; 57:55–64.

82. Nitzan A, Mukamel S, Jortner J. Energy gap law for vibrational relaxation of

a molecule in a dense medium. J Chem Phys 1975; 63:200–207.

83. Chandler DW, Ewing GE. Transfer and storage of vibrational energy in

liquids: liquid nitrogen and its solutions with carbon monoxide. J Chem Phys

1980; 73:4904–4913.

84. Herzberg G. Molecular Spectra and Molecular Structure II. Infrared and

Raman Spectra of Polyatomic Molecules. New York: Van Nostrand Reinhold,

1945.

85. Bai YS, Fayer MD. Time scales and optical dephasing measurements:

Investigation of dynamics in complex systems. Phys Rev B 1989;

39:11066–11084.

86. Matsumoto M, Gubbins KE. Hydrogen bonding in liquid methanol. J Chem

Phys 1990; 93:1981–1994.

87. Liddel U, Becker ED. Infra-red spectroscopic studies of hydrogen bonding in

methanol, ethanol, and t-butanol. Spectrochem Acta 1957; 10:70–84.

88. Graener H, Ye TQ, Laubereau A. Ultrafast vibrational predissociation of

hydrogen bonds: mode selective infrared photochemistry in liquids. J Chem

Phys 1989; 91:1043–1046.

89. Graener H, Ye TQ, Laubereau A. Ultrafast dynamics of hydrogen bonds

directly observed by time-resolved infrared spectroscopy. J Chem Phys 1989;

90:3413–3416.

90. Laenen R, Rauscher C. Time-resolved infrared spectroscopy of ethanol

monomers in liquid solution: molecular reorientation and energy relaxation

times. Chem Phys Lett 1997; 274:63–70.

91. Laenen R, Rauscher C, Laubereau A. Vibrational energy redistribution of

ethanol oligomers and dissociation of hydrogen bonds after ultrafast infrared

excitation. Chem Phys Lett 1998; 283:7–14.

92. Woutersen S, Emmerichs U, Bakker HJ. A femtosecond midinfrared pumpprobe study of hydrogen-bonding in ethanol. J Chem Phys 1997;

107:1483–1490.

93. Zhao W, Wright JC. Doubly vibrationally enhanced four-wave mixing-the

optical analogue to 2D NMR. Science, Phys Rev Lett 2000; 84:1411–1414.



Copyright © 2001 by Taylor & Francis Group, LLC



592



Iwaki et al.



94. Tokmakoff A, Fleming GR. Two-dimensional Raman spectroscopy of the

intermolecular modes of liquid CS2 . J Chem Phys 1997; 106:2569–2582.

95. Rector KD, Fayer MD, Engholm JR, Crosson E, Smith TI, Schwettman HA.

T2 selective scanning vibrational echo spectroscopy. Chem Phys Lett 1999;

305:51–56.

96. Deak JC, Iwaki LK, Rhea ST, Dlott DD. Ultrafast infrared-Raman studies

of vibrational relaxation in polyatomic liquids. J Raman Spectrosc 2000;

31:263–274.



Copyright © 2001 by Taylor & Francis Group, LLC



14

Coulomb Force and Intramolecular

Energy Flow Effects for Vibrational

Energy Transfer for Small Molecules

in Polar Solvents

James T. Hynes

University of Colorado, Boulder, Colorado, and Ecole Normale Sup´erieure,

Paris, France



Rossend Rey

Universitat Polit`ecnica de Catalunya, Barcelona, Spain



I. INTRODUCTION



Molecular vibrational energy transfer (VET) in solution is a phenomenon of

long standing and — as the present volume attests — continuing interest

and importance (1–11). In this chapter, we discuss two aspects of solution

phase VET: (1) the role of electrostatic Coulomb forces and (2) polyatomic

VET, i.e., the involvement of noninitially excited vibrational modes in a

polyatomic in the vibrational relaxation. While perhaps not lying currently

in the mainstream of theoretical effort in solution phase VET — which

is often focused on diatomics immersed in simple fluids — these topics

represent rivulets that we anticipate will soon emerge in full flood, as more

complex molecular solutes and solutions come under increasing modern

experimental and theoretical scrutiny.

Both of these topics are readily motivated, since obviously most

vibrating bonds or modes are polar to some degree and most common



Copyright © 2001 by Taylor & Francis Group, LLC



594



Hynes and Rey



molecular solvents are polar, and most molecules are polyatomic. (But we

should hasten to add that in this chapter, “polyatomic” will usually turn

out to mean triatomic, a first foray into the area which opens a window on

the possibilities but which remains within the reach of current theoretical

and (detailed) experimental probes.) However, there are further aspects to

the motivation, here illustrated by just two examples. First, to the degree

that the Coulomb force issue is important, the isolated binary collision

ideas so fruitful in simple systems (1,3,4,6) will need to be replaced, since

long-ranged solute-solvent interactions will certainly differ in character

from short-range binary collisions. Second, the polyatomic aspect brings

in the question — pervasive in chemical dynamics — of the competition

of intramolecular versus intermolecular energy transfer. Indeed, an alternate

title for this topic could be intramolecular vibrational redistribution (IVR)

in solution, although we will mainly focus on this topic at low excitation

energies, rather than the higher energies more commonly associated with

IVR. And while chemical reactions in solution are beyond the scope of this

chapter, contemplation of the microscopic level course of a paradigm solution reaction such as the SN 2 reaction between chloride ion and the methyl

chloride molecule in water solvent (12) quickly shows that these two VET

topics must be comprehended if we are ever to have a detailed molecular

level description of the pathway and dynamics of solution reactions. As a

further connection to reactions in solution, it is evident that the issues of the

present chapter lie at the very heart of the ability to induce, and to induce

in any selective way, such reactions (13–15). Finally — but assuredly not

the least of motivations — assorted advances in experimental techniques

are following the probing of a variety of molecular systems in which the

issues of this chapter come to the fore.

It should also be frankly acknowledged here that there are a

variety of theoretical challenges associated with these problems that are

not highlighted at all in this chapter. These range from formulation

questions involving quantum versus classical issues in calculating rates

(see, for example, Chapter 16) to the quantum chemical electronic structure

issues of solute intramolecular force fields. These and other difficulties

certainly impede the theoretical ability to confidently predict VET rates

and mechanisms, but not the desire to try.

The outline of this chapter is as follows. In Section II we treat the

Coulomb force issue, progressing from a polar molecule in water to the

case of the cyanide ion in water, concluding with a brief discussion of

the special effects arising when the solute charge distribution is not fixed.

In Section III we deal with the polyatomic issue, for which much less



Copyright © 2001 by Taylor & Francis Group, LLC



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

G. Spectral Evolution in Associated Liquids

Tải bản đầy đủ ngay(0 tr)

×