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Length dependent resolved pseudo-Raman spectra of PGA's
Volume 45A, number 3
PHYSICS LETTERS
24 September 1973
LENGTH DEPENDENT RESOLVED PSEUDO-RAMAN SPECTRA OF PGA’S J.P. BISCAR and N. KOLLIAS Department of Physics, The University of Wyoming, Laramie, Wyoming 82071, USA Received 3 August 1973 The resolved pseudo-Raman spectra of Poly-L-Glutamic acid, long chain polymers, are directly related to the chain length.
The relation between the length of a polymer chain and its resolved pseudo-Raman broad band is presented for the first time. The optical set up used in these experiments is the same as the one previously described [1] to obtain a reliable broad band of PolyL-Glutamic Acid (PGA). We have been able to resolve, in a series of wide peaks, the non-fluorescent bands of PGA. polymers varying in molecular ~veight from 40000 to 200 000. The experimental data show that, the position of these resolved wide peaks, in Raman scale is directly related to the chain length. It is possible to distinguish the resolved spectra as presented here, of PGA polymers having molecular weights as close as 93 000 and 103 000. These data are obtained with the same freeze dry “molecular individualization” technique described in [2]. In ref. [2] the resolved spectrum of PGA (103 000) contains also the spectrum of dimers(2 X 103 000) with a periodicity of 430 cm~which was intentionally shown. The latter broadens the spectrum of the 103 000 chains and makes the position of their center more uncertain. For the present data the freeze dried sample was warmed up under vacuum to 40°C,by reversing the current in the thermoelectric element. As a result the contribution of the dimers to the spectrum was minimized while the main peaks became sharper and larger. Each molecular produces a series of peaks which are the harmonics of a corresponding fundamental. The fundamental for 103000 PGA is 860 ±15 cm’ (X = 11.62 im) and for the 93 000 PGA is 950 ±15 cm1 (X = 10.52 jim). These spectra are shown in fig. 1. It is possible to determine the position of the fifth harmonic within 150 cm1 providing enough precision for the corresponding fundamental. The ratio of the molecular weights for the two molecules is 1.10 the same as the ratio of the vacuum
LASER 5145 A
PGA
Mw 93,000
950
I MW
hi
ii IIII
03,000
liii
I
I iii
iii
iii,,
I
2
ii
I
III 11111111
III II
860
ii
Ii
I
I iiliiiiiiii
II
I
till’’’ iii
3
III
4
iii
5*1000
WAVENUMBER (Cnr’)
Fig. 1. The positions of the resolved pseudo-Ramanpeaks for PGA molecules of 93 000 and 103 000 molecules weight. The fundamental frequencies are respectively 860 and 950 cm
wavelengths corresponding to the fundamentals of their resolved pseudo-Raman spectra. Since the chain lengths for the two PGA’s are proportional to the molecular weight, the present data provide experimental evidence that the wavelength of the fundamental is proportional to the molecular chain length. This remains true at least up to 200 000 molecular weights because the dimers (2 X 103 000) produced a spectrum of 430 cm’ fundamental, as expected by extrapolating the data of fig. 1. A hypothesis has been advanced [3] taking into account this remarkable fact that the resolved pseudo-Raman spectrum is directly related to the polymer chain length. It is known that the substitution of an atom of hydrogen, at one end of a molecular chain by e.g. a fluorine atom, modifies the electronic density along the chain. One can predict the dynamics of the same phenomenon. If the electronic density perturbation is produced for a short time (~ 10—14 sec) one could determine its velocity of propagation V~,along the bounding electron chain. Since these are valence electrons, and not conduction or free electrons, the velocity of propagation will be slow and such that 191
Volume 45A, number 3
Po Vp
PHYSICS LETTERS
C;
(1)
P
0 is the index of propagation; C is the velocity of light in vaccum. In cases where its lifetime is larger than the time for the perturbation to propagate back and forth along the chain, after reflection at its ends, standing waves are possible for periodical excitations. They will take place with frequencies for which the whole chain oscillates such that for a chain length L we have:
P0 L
=
4nX
(2)
,
1,2,3,...; n = 1 gives the fundamental, followed by harmonics if there is no dispersion (P(X) = F0). This gives rise to discrete frequencies or Molecular Resonance energy levels. By scattering, some laser photons will lose energy to these molecular standing waves producing Stokes and anti-Stokes frequencies w5 and WAS n
24 September 1973
a very strong argument in favor of this Molecular Resonance. It is experimentally established that these resolved wide peaks have a Raman-like behavior; they remain in the same position, in Raman scale, for different exciting frequencies. A PGA chain of 93 000 molecular weight has 632 Glutamic Acid molecules having [4] each a length of 4.5 A. The length (even folded) of the whole chain is thus 2844 A. We have found for this molecule that the fundamental frequency was at 950 cm~’(or X = 10.52 pm). From these values eqs.(2) and (1) give: P0 = 18.69 and V~= 16 213 km/sec. This hypothesis predicts the whole series of resolved wide peaks. It predicts also why a solvent, as water, dampens the whole broad band by its IR absorption effect and also shifts the resonance freqiencies down by its dielectric effect [5] We have obtained similar data in several different molecules [6] where this .
=
“~‘L
~‘~M
WAS = °~L +WM ;
(3)
is the frequency of the laser; WM represents here all molecular frequencies calculated from eq. (2). The main distinction with the normal Raman effect is that this hypothesis considers a purely electronic oscillator, involving the whole chain, without motion of the nuclei as is the case for the mechanical type of oscillators of the normal molecular Raman effect. This pseudo-Raman scattering could be better labeled: “Electromagnetic Molecular Electronic Resonance Scattering”. This model requires good boundary reflection to provide proper frequencies and avoid that the perturbation propagates to adjacent molecules. It is an experimental fact that the resolved wide peaks appear only when the chain molecules are “individualized” This sine qua non condition, which also provides the major difficulty in resolving these spectra, is by itself WL
192
hypothesis predicts well the resolved spectrum. The final explanation of the new experimental data reported here remains however open to other possible theories.
References [1] J.P. Biscar and N. Kollias, Phys. Lett. 44A (1973) 373. [2] J.P. and N. Kollias, Phys. Lett.Phys. 46A Soc. (1973) [3] J.P. Biscar Biscar and N. Koliias, Bull. Am. 18 189. (March 1973) 373, DC3. [41 L. Pauling, The nature of the chemical bond (3rd ed., Cornell University Press, Ithaca, New York, 1960) p.498. [51 Biscar and and J.P.Biscar, N. Koilias, Bull. to beAm. published. [6] J.P. N. Kollias Phys. Soc. 18 (March 1973) 373, DC4.
PHYSICS LETTERS
24 September 1973
LENGTH DEPENDENT RESOLVED PSEUDO-RAMAN SPECTRA OF PGA’S J.P. BISCAR and N. KOLLIAS Department of Physics, The University of Wyoming, Laramie, Wyoming 82071, USA Received 3 August 1973 The resolved pseudo-Raman spectra of Poly-L-Glutamic acid, long chain polymers, are directly related to the chain length.
The relation between the length of a polymer chain and its resolved pseudo-Raman broad band is presented for the first time. The optical set up used in these experiments is the same as the one previously described [1] to obtain a reliable broad band of PolyL-Glutamic Acid (PGA). We have been able to resolve, in a series of wide peaks, the non-fluorescent bands of PGA. polymers varying in molecular ~veight from 40000 to 200 000. The experimental data show that, the position of these resolved wide peaks, in Raman scale is directly related to the chain length. It is possible to distinguish the resolved spectra as presented here, of PGA polymers having molecular weights as close as 93 000 and 103 000. These data are obtained with the same freeze dry “molecular individualization” technique described in [2]. In ref. [2] the resolved spectrum of PGA (103 000) contains also the spectrum of dimers(2 X 103 000) with a periodicity of 430 cm~which was intentionally shown. The latter broadens the spectrum of the 103 000 chains and makes the position of their center more uncertain. For the present data the freeze dried sample was warmed up under vacuum to 40°C,by reversing the current in the thermoelectric element. As a result the contribution of the dimers to the spectrum was minimized while the main peaks became sharper and larger. Each molecular produces a series of peaks which are the harmonics of a corresponding fundamental. The fundamental for 103000 PGA is 860 ±15 cm’ (X = 11.62 im) and for the 93 000 PGA is 950 ±15 cm1 (X = 10.52 jim). These spectra are shown in fig. 1. It is possible to determine the position of the fifth harmonic within 150 cm1 providing enough precision for the corresponding fundamental. The ratio of the molecular weights for the two molecules is 1.10 the same as the ratio of the vacuum
LASER 5145 A
PGA
Mw 93,000
950
I MW
hi
ii IIII
03,000
liii
I
I iii
iii
iii,,
I
2
ii
I
III 11111111
III II
860
ii
Ii
I
I iiliiiiiiii
II
I
till’’’ iii
3
III
4
iii
5*1000
WAVENUMBER (Cnr’)
Fig. 1. The positions of the resolved pseudo-Ramanpeaks for PGA molecules of 93 000 and 103 000 molecules weight. The fundamental frequencies are respectively 860 and 950 cm
wavelengths corresponding to the fundamentals of their resolved pseudo-Raman spectra. Since the chain lengths for the two PGA’s are proportional to the molecular weight, the present data provide experimental evidence that the wavelength of the fundamental is proportional to the molecular chain length. This remains true at least up to 200 000 molecular weights because the dimers (2 X 103 000) produced a spectrum of 430 cm’ fundamental, as expected by extrapolating the data of fig. 1. A hypothesis has been advanced [3] taking into account this remarkable fact that the resolved pseudo-Raman spectrum is directly related to the polymer chain length. It is known that the substitution of an atom of hydrogen, at one end of a molecular chain by e.g. a fluorine atom, modifies the electronic density along the chain. One can predict the dynamics of the same phenomenon. If the electronic density perturbation is produced for a short time (~ 10—14 sec) one could determine its velocity of propagation V~,along the bounding electron chain. Since these are valence electrons, and not conduction or free electrons, the velocity of propagation will be slow and such that 191
Volume 45A, number 3
Po Vp
PHYSICS LETTERS
C;
(1)
P
0 is the index of propagation; C is the velocity of light in vaccum. In cases where its lifetime is larger than the time for the perturbation to propagate back and forth along the chain, after reflection at its ends, standing waves are possible for periodical excitations. They will take place with frequencies for which the whole chain oscillates such that for a chain length L we have:
P0 L
=
4nX
(2)
,
1,2,3,...; n = 1 gives the fundamental, followed by harmonics if there is no dispersion (P(X) = F0). This gives rise to discrete frequencies or Molecular Resonance energy levels. By scattering, some laser photons will lose energy to these molecular standing waves producing Stokes and anti-Stokes frequencies w5 and WAS n
24 September 1973
a very strong argument in favor of this Molecular Resonance. It is experimentally established that these resolved wide peaks have a Raman-like behavior; they remain in the same position, in Raman scale, for different exciting frequencies. A PGA chain of 93 000 molecular weight has 632 Glutamic Acid molecules having [4] each a length of 4.5 A. The length (even folded) of the whole chain is thus 2844 A. We have found for this molecule that the fundamental frequency was at 950 cm~’(or X = 10.52 pm). From these values eqs.(2) and (1) give: P0 = 18.69 and V~= 16 213 km/sec. This hypothesis predicts the whole series of resolved wide peaks. It predicts also why a solvent, as water, dampens the whole broad band by its IR absorption effect and also shifts the resonance freqiencies down by its dielectric effect [5] We have obtained similar data in several different molecules [6] where this .
=
“~‘L
~‘~M
WAS = °~L +WM ;
(3)
is the frequency of the laser; WM represents here all molecular frequencies calculated from eq. (2). The main distinction with the normal Raman effect is that this hypothesis considers a purely electronic oscillator, involving the whole chain, without motion of the nuclei as is the case for the mechanical type of oscillators of the normal molecular Raman effect. This pseudo-Raman scattering could be better labeled: “Electromagnetic Molecular Electronic Resonance Scattering”. This model requires good boundary reflection to provide proper frequencies and avoid that the perturbation propagates to adjacent molecules. It is an experimental fact that the resolved wide peaks appear only when the chain molecules are “individualized” This sine qua non condition, which also provides the major difficulty in resolving these spectra, is by itself WL
192
hypothesis predicts well the resolved spectrum. The final explanation of the new experimental data reported here remains however open to other possible theories.
References [1] J.P. Biscar and N. Kollias, Phys. Lett. 44A (1973) 373. [2] J.P. and N. Kollias, Phys. Lett.Phys. 46A Soc. (1973) [3] J.P. Biscar Biscar and N. Koliias, Bull. Am. 18 189. (March 1973) 373, DC3. [41 L. Pauling, The nature of the chemical bond (3rd ed., Cornell University Press, Ithaca, New York, 1960) p.498. [51 Biscar and and J.P.Biscar, N. Koilias, Bull. to beAm. published. [6] J.P. N. Kollias Phys. Soc. 18 (March 1973) 373, DC4.