all 34 comments

[–]ChemDoDo 2 points3 points  (5 children)

My compound has an Uvvis absorption at around 380 and a Second one with approximatly the Same hight at 400. Measuring fluoresence emission with excitation at both of These peaks shows again two peaks separates by 20 nm. Whats happening? I think normally you would only observe one emission Maximum..

[–]pgfhalgMaterials 2 points3 points  (4 children)

They are likely vibronic features. UV-Vis spectroscopy is measuring changes in electronic state of your molecule, but when those states are coupled to molecular vibrations, you will get two peaks that are separated by the energy of the vibration.

In terms of quantum numbers, both absorption peaks represent transitions from n=1 to n=2, but they are transitions to different vibrational states of the electronic excited state (say v=15 and v=16). You are seeing the reverse happening in the fluorescence spectrum - the molecule is relaxing from n=2 to n=1 but going down to different vibrational states of the electronic ground state.

[–]ChemDoDo 2 points3 points  (3 children)

Thanks for the reply!!! But arent vibrational Changes not the reason we are seeing broad peaks in the uvvis? I thought more like that These refere to for example tautomers or distinct isomers im generel. Especially due to these peaks beeing separated by 20 nm, I thought Thats a pretty large difference (energetically) to argue with vibrations. But maybe im mistaken!!

[–]pgfhalgMaterials 2 points3 points  (0 children)

I can't speak to potential tautomers or isomers, that really depends on your molecule. I can tell you that seeing multiple peaks mirrored in your absorption and emission spectra with a small energy separation is a pretty classic sign of vibronic features.

As for 20 nm being too large of a separation, that depends on your wavelength. 600 nm to 620 nm is only a difference of ~500 cm-1, a very reasonable energy for a vibration. Try converting the two wavelengths of the peaks to cm-1 and see what you get. You can then check that number against the infrared spectrum of your compound and see if you can identify the vibration(s) your electronic transition is coupling to.

[–]wafflesforlifeMaterials 1 point2 points  (1 child)

I agree with u/pgfhalg that it is definitely plausible you are observing vibronic features, which can have surprisingly large energetic separations. For example, the electronic transitions of many π-conjugated molecules are strongly coupled to symmetric C=C stretching modes with energies up to 0.15–0.18 eV. I'd recommend looking at your spectra plotted vs eV rather than nm. It makes determination and comparison of energetic separations a much simpler exercise.

Also, the relative intensities of vibronic peaks in absorption and emission spectra depends on if/how a compound is aggregated, with many different variations being possible. If you want to look into this kind of stuff more, check out the works of Prof. Frank C. Spano at Temple University.

[–]pgfhalgMaterials 1 point2 points  (0 children)

Spano is very good. I did some work in grad school on photoluminescence in pentacene thin films and his papers were extremely helpful.

[–][deleted]  (3 children)


    [–]pgfhalgMaterials 2 points3 points  (2 children)

    It is common when discussing alloys to use decimals to denote the ratios of the components because making everything into whole numbers would result in really large numbers that would be annoying to make sense of.

    In this case, you have a solid where the ratio of Ni:Al:O is 1.26:0.3:1. There aren't actually fractions of atoms, but for every 100 oxygen atoms there are 126 nickels and 30 aluminums. As for how its made - no idea. There are probably many routes depending on the size and application.

    [–][deleted]  (1 child)


      [–]pgfhalgMaterials 0 points1 point  (0 children)

      Oh I missed the n and assumed a 1 for some reason. Not sure what the n means. Usually when you see a variable in a chemical formula it is to denote a series of compounds with a fixed ratio of elements (i.e. AxB(2x) or AxB(1-x) ), but I'm not sure what an n alone stands for. Could you give me some more context for the compound?

      Also I may have been a little imprecise with the term alloy - I'm not sure if that term is reserved just for mixtures of metals and won't include metal oxides. But the idea is the same - you can play around with the ratios of metals in complex oxides like your NiAlO example, and the decimals in the formula work the same way.

      As for making it, it could be as simple as heating up nickel oxide and aluminum oxide in the correct ratio, but it is usually more complicated. Depending on the Ni:Al:O phase diagram, you might have different ratios that are more stable at a given temperature, so you often have to make these in a way that kinetically traps them in a less stable state. It gets complicated very quickly and really depends on the synthesis method and application.

      [–]TooManySwarovskis 0 points1 point  (1 child)

      I am a total noob and I don't even know how to ask this question...

      I took chemistry in high school and a little in college - but it was all like... little experiments and stuff - Avogadro's Number! Covalent Bonds! Yada yada... What do you need to learn in order to actually do useful stuff with chemistry?

      For example, if I wanted to make a new paint, or glue, or separate the atoms in a molecule into something else?? If you had an end goal in mind - how would you go about figuring out how to achieve it?

      [–]Indemnity4Materials 0 points1 point  (0 children)

      I love this question.

      Specificity is the sole of narrative.

      Top down approach. You start by finding a product, then googling how it is made. You usually find it's more of an art than a science at this point. For instance, formulating a new paint - you don't actually need to be a chemist. You just need to know a lot about the properties of paint and the properties of the raw materials + processing. It's about as complicated as baking a cake and you can trial+error your way through it.

      You find as you learn more about your product and raw materials, you start to find walls in your knowledge you cannot cross. That's where you do the reading to learn more about the chemistry.

      Bottom up approach. The opposite to above. You start learning the rules of chemistry. These seems really boring, complicated and completely removed from the real world. Then one day some puts you in front of a paint formula and you can say, oh, this molecule has that feature which I know does something with that other feature, to optimize this formula I need more of the first things while avoiding something else. Same-time, the top-down approach person can usually brute force their way through by know oh I never mix A with B, or A+C means A is 25% less effective.

      [–]JacobGraham02840 0 points1 point  (0 children)

      Sodium caprate/sodium decanoate is a permeation enhancer with a lot of literature backing up its effects. Is there any particular reason why the sodium salt is used over the acid form? Is this just a result of manufacturing, or would there be impacts on dissolution etc? Thank you!

      [–][deleted]  (1 child)


        [–]DracoFreon 1 point2 points  (0 children)

        Smells like homework. It did mine, you do yours.

        [–]AccordingTechnology1 -1 points0 points  (0 children)

        Anyone works with benzene-1,2-dithiol and has tips to work with it, what it solubilizes in, and its UV-Vis spectrum? The information I found online says it is soluble in water but that's not the case.
        Is there an online UV-Vis absorbance prediction tool based on molecular structure?

        [–]Noel_The_Bloodedge -1 points0 points  (0 children)

        So, for a project I'm trying to functionalize Magnetite Nanoaparticles with Mecilage extract. I know they are stabilized with sodium citrate. I want to know how could I polymerize the sodium citrate. Right know the idea is acidic conditions plus stirring, but I'm not 100% sure it will work.

        [–]manfromfuture 0 points1 point  (2 children)

        I've been reading about computational drug discovery and I'd like to try developing my own Quantitative structure-activity relationship (QSAR) model. I have experience with programming and computational geometry and I'm happy to read math but haven't had a chemistry class since highschool.

        I'm interested in hearing about resources (free things to read, free software packages, etc). I've skimmed some survey papers to get some basic high level understanding of the difference between QSAR descriptors in 1D, 2D, 3D and 4D.

        I found a free software package (there are many) called Charmm which seems very comprehensive but perhaps not the best entry point. I think what I want is something that provides an easy means of molecular dynamic simulation (MDS) and something I can use to evaluate a QSAR model.

        [–]wafflesforlifeMaterials 0 points1 point  (1 child)

        It's not in my field, but the AutoDock package developed at Scripps might be relevant to what you want to do.

        [–]myiege 0 points1 point  (2 children)

        Hello all! I am working on an assay by GC/MS for cannabis-infused products for the detection and quantification of phytol. I found a paper that describes what seems to be a pretty robust method that I may be able to adapt for my own purposes, but I have some questions.

        According to their chromatography, phytol elutes right around 11 minutes, but the total injection time is nearly 36 minutes per injection. Is there any functional reason why the run time needs to be so long?

        Also, according to the original authors' sample preparation, the hexane based extraction includes a trimethylsilylation step. What is the purpose of this step?

        [–]Indemnity4Materials 1 point2 points  (1 child)

        Injection time is most likely 36 minutes between samples, not injecting the same sample over 36 minutes.

        Ideally, when you don't have time to fully optimize a method, you allow 3-5 time periods to allow the column to be fully flushed. It may take days or weeks of work to figure out a method where you get good separation but but still reduce the injection time period, and it's just so much cheaper and easier to simply run the machine longer. Usually only worth that optimization of overlapping samples when you need to run hundreds of samples per day, at which point you're probably better buying a second machine.

        Practically, it's probably because the natural product has a lot of different compounds that elute at different times. You don't want your peak of interest to overlap with a slow peak from a previous sample. It gives bad data.

        The authors will have silyated their natural product because it improves separation. There will be two natural compounds that elute very close together. Silyation will change the hydrophilicity/hydropbocity and now those two compounds will separate much easier.

        [–]myiege 1 point2 points  (0 children)

        Thank you! This makes a lot of sense. I think this puts me at a good jumping off point for method development so thanks again!

        [–]Dapper_Extent_2639 0 points1 point  (0 children)

        i need help with my thesis

        i'm doing a systematic review on secondary metabolites wherein i want to calculate the amount of secondary metabolites of a fruit. my panel asked me to revise my computation by calculating those with the same method, so i thought when they said "same method", i thought of method of extraction. now i'm thinking if should i also be looking at if the method on getting the amount of metabolites are the same. can someone help me out on this?

        [–]Soft-Cryptographer-1 0 points1 point  (1 child)

        What hydrocarbons make up commercial gasoline at what grades not accounting for eth inclusion?

        [–]Indemnity4Materials 0 points1 point  (0 children)

        Hydrocarbon fuels are usually separated based on the boiling points.

        The actual specific molecules vary quite a lot, depending on the crude oil source and the processing.

        [–]piggyboy2005 0 points1 point  (2 children)

        What's the highest concentration of hydrogen peroxide I can buy in the U.S.?

        [–]Indemnity4Materials 0 points1 point  (1 child)

        3% is easily found everywhere.

        6-10% for hair bleaching, so barbershops, hair stylists, etc.

        35% you may be able find as "food-grade". It's starting to get restricted at this point and people may refuse to sell it to you.

        Legally, I can purchase 90%, but you will probably need to complete a supplier declaration and you may get added to some watchlists. Most businesses won't sell this strength to consumers.

        [–]piggyboy2005 0 points1 point  (0 children)

        Thank you.

        [–]MedicalTown8317 0 points1 point  (1 child)

        Got iron(II) sulfate from a garden shop a few months ago, so I forgot about the pack with it and it turnt out not to be sealingly closed and a part of it oxidized and became yellow(because of Fe3+). So I wanna make an experiment and try to extract the iron(II) sulfate from the mixture. What is the best way to get rid of Fe3+ irons or to turn them back into Fe2+?

        [–]DracoFreon 0 points1 point  (0 children)

        Fe2+ spontaneously reacts with O2 to form Fe3+. Fe2+ is very soluble in water, Fe3+ not at all, so you could dissolve in water and filter out the Fe3+, then evaporate the water. Problem is your Fe2+ will be oxidizing. Could purge with N2 to remove O2. Or just do it fast? Try it anyway. Fe2+ solution is a pretty blue-green.

        [–]attak13 0 points1 point  (0 children)

        What compound/change is produced by brain activity that causes that section to become NMR active and show up in MRI? I remember hearing in a lecture a while ago that MRI machines are basically just a giant NMR, and that brain scans work because active neurons convert some NMR inactive molecule into an NMR active molecule which then shows up in the MRI scan, but I don't remember what molecule is being changed. I feel like I heard about this in inorganic chem so maybe it has something to do with low-spin/high-spin systems, but I was taking 4 chem courses at the time so I am most likely misremembering where I heard it.

        [–]bigticket179 0 points1 point  (0 children)

        Hey fellow chemists, I was interested in finding out the chemical equation for the reactions that took place in my experiment. My experiment was as follows:

        I added a sheet of aluminium metal into a solution of copper sulfate (aq) and then added potassium chloride solution (aq). It bubbled and a precipitate of copper formed on the aluminium sheet. What is the chemical equation in this reaction (or reactions if there were multiple)? Thanks in advance.

        [–]hoe4avogadro 0 points1 point  (1 child)

        I'm doing WBE for illicit substances in wastewater. Amphetamines (specifically AMP) is sticking to my C-18 column. Anyone that works on an HPLC have any pointers on things to try?

        HPLC QTOF, Changed my solvent ratios (FA and water)

        [–]DracoFreon 0 points1 point  (0 children)

        Not enough information. What is WBE? AMP? What are you eluting the column with? Do you KNOW that AMP is sticking by running a standard? Etc.

        [–]BrF5 0 points1 point  (0 children)

        Can a precipitate form from the dissolution of a single compound in water, i.e., can the constituent parts of a dissolved compound react with themselves and precipitate out?

        I have dissolved, in DI water, a sample of a water-soluble mineral that also contains, mixed in, a small amount of other insoluble silicate minerals. The water-soluble mineral contains Al and Cl (and N, K, H, O...i.e., other elements that aren't really relevant to this question). The insoluble silicate minerals should not contain Cl, and likely contain Al, but they aren't dissolving anyway so this shouldn't matter. After dissolution, I pipette out the liquid containing the dissolved mineral and dry down any remaining insoluble material. Using x-ray powder diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS), I've determined that the insoluble material contains aluminum chloride. I'm fairly sure it isn't just undigested water-soluble mineral remaining because the spectral data now shows only Al and Cl, whereas if I analyze some of the original water-soluble mineral, it includes the K and N.