For people to come to conclusion that nuclear strike on Ukraine could lead to capitulation is not unreasonable. But there is enough difference, enough nuance to anticipate that it not only would not lead to capitulation but it would only compound the disaster that is Ukraine for Russia further.
I don't see a way for UKR to capitulate after a nuclear strike in almost any scenario (can someone pose a realistic scenario?? Wipping out the entire UKR?? Could any paper signed unilaterally by Ukraine have any validity??) , as it would trigger an inmediate effect on current world rules and powers. (Nukeing a foreign nation in a war they have started)
Bear in mind that West won't let Ukraine to capitulate, as it affects this time directly to their own security, and dropping a nuclear weapon in Europe would have existential and reasonable doubts about our security. Can Ukraine capitulate? It doesn't matter, RU would be directly at war with Europe. Despite the initial fear, europeans doesn't want to live under the fear, and more if it's coming from RU, there are enough nuclear and non-nuclear weapons here to retaliate.
I think the scenario for Russia after launching a nuclear device in or nearby Ukraine, would lead to an inmediate triggering of several international implications and an extraordinary UN security council meeting with a lot of dooming, stockmarkets falling as hell and a news around the world blaming RU about any bad thing, including their allies after some days of uncertainty.
Key here is what is giving some oxigen to Russia is China and India, both nuclear powers. Yes I see them blocking a nuclear attack as a response by US.
But the tacit agreement that if Russia uses the weapon again, it would lead to a reataliation from several sides and contexts, even nuclear. That should prevent Russia to use it again, and giving time for the rest of the world to close the fence around current Moscow Elites.
Just think also on what common Russia population would do if (I don't know the exact number) more than a million? people left the country just thinking on being drafted for the war. What this could lead if the Russian people is under the thought of international community wondering about of a nuclear retaliation attack on them and where, after a war they started on foreign lands? What Belarus population would do with Luka for example? Can we see some significat RU army elements detaching from current goverment orders? I believe the fear effect would change from the West to RU itself and the sense of power from RU to the West, as we will be just wondering how to retaliate in any posible way including from many different countries, while RU elites blaming West every time with less reason.
That could lead to consecuences like US launching a conventional attack over a RU strategic asset like the Black sea fleet or completely destroy the Sevastopol base, which would negate one of the reasons for wanting Crimea, maybe striking some industrial sites at night near a big city? Or directly giving nuclear weapons to UKR. Putting even more diplomatic preassure on Moscow elites, maybe start open conversations with oligarchs ( who are going to adquire significant private security services and act outside national powers ), promote internal protests, talks with poor/secondary regions about independence and even further on helping those regions having nuclear weapons to change their firing control for being able to fire them by themselves, and giving security assurances to them.
We shall not forget that Putin unilaterally anexed Crimea, Dombas, Zaporiyia and Kherson, so he can be striked back with his own tactics, but this time even with simultaneous international recognition. What Manchuria ppl wants, what China doing? Are we going to see Khadirov fleeing from Chechnya? What Ru did on foreign democratic elections like in the US? Plenty of room here.
At the end all what West were doing until now was to provide UKR with weapons for being able of defending themselves, as some treaties signed by Russia states. Propaganda would not have any sense and they would be seen as the offenders who started the conflict, entering hard on the blaming game and openly arising the question if such government should exist.
I see that scenario even worse than detonating a nuclear weapon on its own territory.