Communication pubs.acs.org/jchemeduc
The Particle in a Box Redux: Improved Experimental Conditions for the Laser Synthesis of Linear Polyynes Melanie Henry, Hudson G. Roth, and Bruce D. Anderson* Department of Chemistry, Muhlenberg College, Allentown, Pennsylvania 18104, United States S Supporting Information *
ABSTRACT: The one-dimensional, particle-in-a-box model is an integral part of most undergraduate physical chemistry courses. Linear polyynes can be synthesized and are found to serve as real-world examples of the particle-in-a-box model. An improved set of experimental conditions for the laser synthesis of linear polyynes has been determined. Specifically, a Nd:YAG laser operating at 532 nm and 20 mJ/pulse for 5 min can produce sufficient polyynes to be detected with a UV−vis spectrophotometer. Students can use the experimental wavelength and the eigenvalue expression for the particle in a box to calculate the length of the box and compare it to the length of the molecule with good results. The lower laser energy used here is important because it enables a less costly laser to be used for the experiment, which will enable more instructors to incorporate the experiment into their curriculum. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Alkynes, Laboratory Equipment/Apparatus, Lasers, Quantum Chemistry, UV−Vis Spectroscopy
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n an earlier article,1 linear polyynes were shown to serve as another class of molecules that follow the one-dimensional, particle-in-a-box model along with cyanine dyes2 and diphenyl compounds.3 The experiment described used a high power Nd:YAG laser (532 nm, 80 mJ/pulse) for 60 min to produce linear polyynes in solution. The resulting solution was filtered and a sample was injected into the gas chromatograph−mass spectrometer (GC−MS) to identify which polyynes were present. Finally, a UV−vis spectrum of the filtered solution was acquired and the peak wavelengths were used to calculate the box length for the linear polyynes present in the solution based on the quantum mechanical particle-in-a-box model. The experimental box lengths were then compared to box lengths calculated using Spartan and from tabulated bond length values. Recent work by Matsutani et.al.4 and Shin et al.5 has shown that polyynes can be produced in solution using much shorter irradiation times, as low as 2.5 min. Interestingly, both groups used UV−vis absorbance for detection rather than GC−MS. On the basis of the work of these groups, a simpler procedure for producing linear polyynes to study the particle-in-a-box model as part of an undergraduate course in physical chemistry was developed.
Figure 1. UV−vis absorbance scan of graphite in ethanol lased for 6 min at 20 mJ/pulse.
due to secondary peaks for the polyynes identified, the presence of shorter polyynes (C4H2 and C6H2) in the solution, and the absorbance of ethanol. Higher laser energies, 30 or 40 mJ/pulse (up to 80 mJ/pulse), will produce more intense polyyne peaks in the spectrum with 5 min of irradiation and are recommended for use if the laser is capable of reaching this level. Although the polyynes readily appear in the UV−vis spectrum after 5 min of irradiation, no polyyne signal is seen on the GC−MS for this same sample. Possible explanations for this are that the polyynes have a high molar absorptivity,6 and thus, smaller quantities can be detected using absorbance. Also, whereas samples of polyynes can be seen on the GC−MS as described earlier,1 the peaks are small because the neutral
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NEW CONDITIONS After investigating a variety of concentrations, laser energies, and lasing times, a Nd:YAG laser at 20 mJ/pulse was found to produce easily detectable quantities of polyynes in as little as 5 min. The absorbance data in Figure 1 was acquired for a suspension of graphite in ethanol (6 mg in 2 mL) that was irradiated with 532 nm light at 20 mJ/pulse for 6 min. In the spectrum, a peak for C8H2 is clearly visible at 226 nm, whereas that for C10H2 appears at 252 nm and a small C12H2 peak is seen at 275 nm. The other peaks observed in the spectrum are © 2012 American Chemical Society and Division of Chemical Education, Inc.
Published: April 12, 2012 960
dx.doi.org/10.1021/ed200728k | J. Chem. Educ. 2012, 89, 960−961
Journal of Chemical Education
Communication
Table 1. Comparison of Lasers and Conditions for the Production of Polyynes Laser Characteristic
Nd:YAG
HeCd
Nd:YAG
Nd:YAG
Nd:YAG
Wavelength Pulse length Pulse energy Avg. Power Peak Power Focused Beam diam. Intensity Photons/bond Lasing time Polyynes
532 nm 5 ns 20 mJ N/A 4 MW No 0.8 8 MW/cm2 2.3 6 min Yes
442 nm N/A N/A 10 mW N/A Yes 0.15 0.6 W/cm2 1.9 60 min No
355 nm 5 ns 30 mJ N/A 6 MW No 0.8 12 MW/cm2 1.5 15 min Yes
355 nm 5 ns 5 mJ N/A 1 MW Yes 0.3 14 MW/cm2 1.5 15 min Yes
355 nm 5 ns 1.5 mJ N/A 0.3 MW Yes 0.3 4 MW/cm2 1.5 75 min No
polyynes are difficult to ionize.7 Both factors contribute to the relative lack of sensitivity for polyynes using GC−MS.
effectively separated and identified using HPLC with a multichannel UV−vis spectrometer for detection. In particular, the HPLC would enable students to see a series of peaks for each of the polyynes formed, which facilitates identification of the species present in solution.
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DISCUSSION The advantages of the new experimental conditions are that the reaction can be run in a greatly reduced period of time and that a smaller, less powerful Nd:YAG laser can be used. The reduced reaction time will enable students to perform the experiment more quickly. Because the reaction time is greatly reduced and the need for GC−MS analysis is eliminated, the instructor will have time to incorporate other activities into the experiment, which might include a discussion of lasers, a more detailed computational chemistry analysis, or perhaps combining the polyynes experiment with other particle-in-a-box experiments in a single laboratory period. The lower required laser pulse energy for the reaction will enable instructors at a greater number of schools to implement the experiment because the required laser is more affordable. Other types of lasers and wavelengths were examined to see if polyynes could be successfully generated. The third harmonic (355 nm) of the Nd:YAG laser easily produced polyynes at 30 mJ/pulse with an unfocused beam. Furthermore, a focused 355 nm beam did produce small quantities of polyynes at 5 mJ/ pulse in 15 min, but no evidence of polyyne production was seen with the focused 355 nm beam at 1.5 mJ/pulse even when the irradiation time was increased to 75 min. Lasers not found to produce polyynes are a 10 mW HeCd laser at 442 nm and a 5 W CO2 laser at 10,600 nm. Both of these lasers are continuous. The HeCd laser was focused onto the solution for an 60 min with no evidence of polyyne production. The CO2 laser was focused onto the sample for a couple of minutes, but the ethanol absorbed the laser energy and the solvent boiled off without producing any polyynes. Table 1 summarizes the experimental conditions used. The number of photons required to break an aromatic carbon−carbon bond (518 kJ/mol) in graphite was calculated for each wavelength and shows that a multiphoton process is required for all wavelengths used. Although not included in Table 1, the CO2 laser requires 45.7 photons to break a bond with this energy. The key observation from Table 1 is that pulsed lasers with peak powers greater than or equal to 1 megawatt (MW) and intensities higher than 4 MW/cm2 are required to produce polyynes. Continuous, UV or visible lasers will need much greater powers than those used here to produce polyynes. Another potential experimental improvement that instructors may want to pursue is to analyze the polyyne solution using HPLC. Inoue et.al.8 have shown that the polyynes can be
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CONCLUSION Linear polyynes can be synthesized with a Nd:YAG laser using a lower pulse energy than previously reported. Thus, more institutions will have the capability to perform the experiment and more students will have the opportunity to examine linear polyynes as examples of the particle-in-a-box model.
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ASSOCIATED CONTENT
S Supporting Information *
Instructions for students. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Anderson, B. D.; Gordon, C. M. J. Chem. Educ. 2008, 85, 1279− 1281. (2) Garland, C. W., Nibler, J. W., Shoemaker, D. P. Experiments in Physical Chemistry, 8th ed.; McGraw Hill: New York, 2009; pp 393− 398. (3) Anderson, B. D. J. Chem. Educ. 1997, 74, 985. (4) Matsutani, R.; Kakimoto, T.; Tanaka, H.; Kojima, K. Carbon 2011, 49, 77−81. (5) Shin, S. K.; Song, J. K.; Park, S. M. Appl. Surf. Sci. 2011, 257, 5156−5158. (6) Chalifoux, W. A.; Tykwinski, R. R. Nat. Chem. 2010, 2, 967−971. (7) Wakabayashi, T., Kato, Y., Momose, T., Shida, T., Polyynes: Synthesis, Properties, and Applications; Cataldo, F., Ed.: CRC Press: Boca Raton, FL, 2006; pp 181−195. (8) Inoue, K.; Matsutani, R.; Sanada, T.; Kojima, K. Carbon 2010, 48, 4197−4214.
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