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Neutron quasi-elastic scattering of n-nonadecane
YoIume 62. number 3
NEUI-RON
CHEMICAL PHYSICS LETl-ERS
QUASL-ELASTIC
SCATTERING
1.5April 1979
OF n-NONADECANE
D-H- BONSOR * CT1etm3q Depanntent.
LWwmit_v of Exeter. Exeter EX4 4QD.
J-F- BARRY R IKR E. Aldenmstcn.
Bedshike.
UK
UIS
M-W- NEWBERY, M-V. ShiALLEY Ht_vsicaIC7kwr.ikt~.!_aboratory. Oxforcl.
UK
and
E. GRANZER,
C. KOBERGER,
P-H. NEDWED and J. SCHEIDEL
Received 7 December 1378 A neutron quasi-eIassticinvesti&on of the p&tic phaseof ageement with P simple conrinuous rotaional diffusion model ening is observed in the Iower tempcnture
phase_
A powder sample of 99% pure rr-nonadecane was investigated with the h&x neutron spectrometer on the Pluto reactor st HanveIl [I] - The resolution of the instrument was 200 PeV at an incident waveIength of 4.15 A. Temperature control was simply by means of an zningfepoiselamp, placed cIose to the sample, maintaining a sample temperature of 25 + l°C throughout the experiment_ The plastic-phase transition temperature is 23OC. The sampte was enclosed in a thin aluminium foil envelope, effectively eIiminating container scatter_ The background on the Marx was significant and a bIank run had to be carried out for each neutron momentum transfer(Q) v&e_ The model fitting was carried out at Exeter using an ICL470 computer in double precision Fortran_ The experimental data was processed in a simiIar way to the CIRCA program [?I. used for time-of-flight data, and the experimenta scattering law obtained_ The * Present address:_4pplied Management Science Division, S&on Ltd, S;mderson House. k?emers Street, Londoa WIP 4AQ. UK
576
a randomly orientated srimpleof n-nonadeane shows good of the molecule about its Iong rtxis- No quasi-elastic broad-
mode1 scattering law was convoluted with the instrument resolution and then compared to the experimental scattering law by normalization of peak areas. The rotational diffusion constant D, [3] was varied using the optimization routine VA@SAD from the HarweII subroutine library until a best fit forD, of 0.07 + 0.01 rad/ps(l ps= IO-t2 s) was obtained for al1 Q values simuItaneousIy. The fit is plotted in figs_ I a and 1b. Note the effect due to a slight taiI in the instrument resolution at higher Q_ Since a radius of I.39 a fits the data well, there is no evidence of kinked molecules playing a significant part in the observed broadening. It is feasibie that a smd1 component of translational diffusion exists aiong the moIecular axis. Taking an extreme case, although non-physic+ the data was equaIIy we11fitted by only aIIoGing transIationaI diffusion along the chain axis- A well orientated specimen is required before the component of transIationaI diffusion, if any, can be determined. Such a combination of rotational and translational diffusion has been observed in n-triacosane [4]_
Volume
62, number 3
CHEMICAL
PHYSICS
LETTERS
15 April 1979
(a)
0.0
Energy (tleV:
Energy (MeV)
FI+ l- Satering ktw for n-non;tdeune at ?_PC- The continuous lines are from the uni;rG~l rotation diffusion model, Dr = 0.07 ndlps and 3 r&us of i-39 A. (a) X Experimentd points for Q = 1.0 A-’ _ The dashed line is the instrument resolution_ o Experimental points for Q = 1.75 X-r - (b) X Experimental points for Q = 2 1 A-‘. l Experimental points for Q = 2.3 4-‘ . llle dashed line sh0v.s the difference in allowing for the slight tail of the instrument resolution in the convolution process. hote th;?t in the
quasielastic
WingS,
thee~pcx-imental points fall close together, and some baw been omitted from the plots.
Even allowing for the difference in resolution between the two instruments, Barnes quasi-elastic resuits on a randomly orientated sample of n-nonadecane [5] show substantially more broadening than this work. His value for the rotational correlation time Tt ,X5 ps (~1 = l/O,) at 2S”C is much shorter than our value of 14 ps at 25°C indicating that the molecules are becoming progressively more disordered as the melting point at 3r?“C is approached. There is evidence of such behaviour from various X-ray and NMR studies [G-9] _ The results in the lower temperature phase at 20°C
agree with those of Barnes in that no quasi-elastic broadening is seen on the instrument time scaIe. There-
fore any rotational motion, ue slower than 0.01 rad/ps.
for example,
has a 0, val-
Dr. D. Bloor and Mr. D.J. Ando at Queen Mary College, London, are thanked for the supply of the n-nonadecane sample. We thank our respective institutions for support, allowing us to attend the 1977 Hanvell summer school on neutron scattering, of which this experiment is a part.
577
Volume 62. number 3
CHEMICAL PHYSICS LJX-l-ERS
References [ 11 J-K. Kems and P-A. Reynolds, Prowedings Grenoble 1972 (IAEA. Vienna. 1972) IAEA-.%-I SSlF4[21 A.H. &ton, Atomic Energy Research Establishment. &well Bit5 70. HhlSO. unpublished131 A. Dizu~ous. F_ Volino and H. Hervet. &X01. Phys- 30 (1975) 1181.
578
15 April 1979
[4] D. Bloor. D-H_ Bonsor. D-N. Batchelder and C-G- ~Vind.Qa Mel_ Phys_ 34 (1977) 939[S] J-D_ Barnes. J. Chem. Phys. 58 (1973) 5193[6] W-h!. Mazee. Rec. Trav. Chim. 67 (1948) 197. 171 D.H. Bonsor, Ph. D. Thesis, University of London (1973). [S] IV_ Piesczek. G-R. Strobl and K. i%f&.ahn.Acta Gyst. B 30 (1967) 1278[9 J G_ Strobl. B_ Ewen, E-N’_ Fischer and W. Pies-k. J. Chem. Phys. 61 (1974) 5257.5265.
NEUI-RON
CHEMICAL PHYSICS LETl-ERS
QUASL-ELASTIC
SCATTERING
1.5April 1979
OF n-NONADECANE
D-H- BONSOR * CT1etm3q Depanntent.
LWwmit_v of Exeter. Exeter EX4 4QD.
J-F- BARRY R IKR E. Aldenmstcn.
Bedshike.
UK
UIS
M-W- NEWBERY, M-V. ShiALLEY Ht_vsicaIC7kwr.ikt~.!_aboratory. Oxforcl.
UK
and
E. GRANZER,
C. KOBERGER,
P-H. NEDWED and J. SCHEIDEL
Received 7 December 1378 A neutron quasi-eIassticinvesti&on of the p&tic phaseof ageement with P simple conrinuous rotaional diffusion model ening is observed in the Iower tempcnture
phase_
A powder sample of 99% pure rr-nonadecane was investigated with the h&x neutron spectrometer on the Pluto reactor st HanveIl [I] - The resolution of the instrument was 200 PeV at an incident waveIength of 4.15 A. Temperature control was simply by means of an zningfepoiselamp, placed cIose to the sample, maintaining a sample temperature of 25 + l°C throughout the experiment_ The plastic-phase transition temperature is 23OC. The sampte was enclosed in a thin aluminium foil envelope, effectively eIiminating container scatter_ The background on the Marx was significant and a bIank run had to be carried out for each neutron momentum transfer(Q) v&e_ The model fitting was carried out at Exeter using an ICL470 computer in double precision Fortran_ The experimental data was processed in a simiIar way to the CIRCA program [?I. used for time-of-flight data, and the experimenta scattering law obtained_ The * Present address:_4pplied Management Science Division, S&on Ltd, S;mderson House. k?emers Street, Londoa WIP 4AQ. UK
576
a randomly orientated srimpleof n-nonadeane shows good of the molecule about its Iong rtxis- No quasi-elastic broad-
mode1 scattering law was convoluted with the instrument resolution and then compared to the experimental scattering law by normalization of peak areas. The rotational diffusion constant D, [3] was varied using the optimization routine VA@SAD from the HarweII subroutine library until a best fit forD, of 0.07 + 0.01 rad/ps(l ps= IO-t2 s) was obtained for al1 Q values simuItaneousIy. The fit is plotted in figs_ I a and 1b. Note the effect due to a slight taiI in the instrument resolution at higher Q_ Since a radius of I.39 a fits the data well, there is no evidence of kinked molecules playing a significant part in the observed broadening. It is feasibie that a smd1 component of translational diffusion exists aiong the moIecular axis. Taking an extreme case, although non-physic+ the data was equaIIy we11fitted by only aIIoGing transIationaI diffusion along the chain axis- A well orientated specimen is required before the component of transIationaI diffusion, if any, can be determined. Such a combination of rotational and translational diffusion has been observed in n-triacosane [4]_
Volume
62, number 3
CHEMICAL
PHYSICS
LETTERS
15 April 1979
(a)
0.0
Energy (tleV:
Energy (MeV)
FI+ l- Satering ktw for n-non;tdeune at ?_PC- The continuous lines are from the uni;rG~l rotation diffusion model, Dr = 0.07 ndlps and 3 r&us of i-39 A. (a) X Experimentd points for Q = 1.0 A-’ _ The dashed line is the instrument resolution_ o Experimental points for Q = 1.75 X-r - (b) X Experimental points for Q = 2 1 A-‘. l Experimental points for Q = 2.3 4-‘ . llle dashed line sh0v.s the difference in allowing for the slight tail of the instrument resolution in the convolution process. hote th;?t in the
quasielastic
WingS,
thee~pcx-imental points fall close together, and some baw been omitted from the plots.
Even allowing for the difference in resolution between the two instruments, Barnes quasi-elastic resuits on a randomly orientated sample of n-nonadecane [5] show substantially more broadening than this work. His value for the rotational correlation time Tt ,X5 ps (~1 = l/O,) at 2S”C is much shorter than our value of 14 ps at 25°C indicating that the molecules are becoming progressively more disordered as the melting point at 3r?“C is approached. There is evidence of such behaviour from various X-ray and NMR studies [G-9] _ The results in the lower temperature phase at 20°C
agree with those of Barnes in that no quasi-elastic broadening is seen on the instrument time scaIe. There-
fore any rotational motion, ue slower than 0.01 rad/ps.
for example,
has a 0, val-
Dr. D. Bloor and Mr. D.J. Ando at Queen Mary College, London, are thanked for the supply of the n-nonadecane sample. We thank our respective institutions for support, allowing us to attend the 1977 Hanvell summer school on neutron scattering, of which this experiment is a part.
577
Volume 62. number 3
CHEMICAL PHYSICS LJX-l-ERS
References [ 11 J-K. Kems and P-A. Reynolds, Prowedings Grenoble 1972 (IAEA. Vienna. 1972) IAEA-.%-I SSlF4[21 A.H. &ton, Atomic Energy Research Establishment. &well Bit5 70. HhlSO. unpublished131 A. Dizu~ous. F_ Volino and H. Hervet. &X01. Phys- 30 (1975) 1181.
578
15 April 1979
[4] D. Bloor. D-H_ Bonsor. D-N. Batchelder and C-G- ~Vind.Qa Mel_ Phys_ 34 (1977) 939[S] J-D_ Barnes. J. Chem. Phys. 58 (1973) 5193[6] W-h!. Mazee. Rec. Trav. Chim. 67 (1948) 197. 171 D.H. Bonsor, Ph. D. Thesis, University of London (1973). [S] IV_ Piesczek. G-R. Strobl and K. i%f&.ahn.Acta Gyst. B 30 (1967) 1278[9 J G_ Strobl. B_ Ewen, E-N’_ Fischer and W. Pies-k. J. Chem. Phys. 61 (1974) 5257.5265.